chapter 24

For some communicable diseases, global eradication can be achieved through vaccination, as has occurred for smallpox. Why have no other communicable diseases been eradicated? First, candidates for eradication must satisfy the following criteria:

1)Long-term immunity can be produced through vaccination. 2)Only a few strains exist that need to be considered for vaccine design. 3)Only one significant reservoir of the pathogen exists.

Chloramphenicol

23 S rRNA of 50S ribosome subunit

selective toxicity

A drug that kills harmful microbes without damaging the host

_________________ binds the 23S rRNA of the 50S subunit, interfering with peptidyl transferase activity of the ribosome, which results in the prevention of peptide bond formation. It has a broad spectrum range, but causes a rare and unpredictable form of aplastic anemia, produced from the destruction of the bone marrow progenitor cells that form erythrocytes and leukocytes. The condition is usually fatal unless a bone marrow transplant is performed, which has long-term complications of its own. ___________________ is a last-chance drug used only in serious infections when other antibacterial drugs are not effective.

Chloramphenicol Chloramphenicol

Quinoline

Inhibition of heme detoxification target: Hemozonin

Antibiotics- the beginning

• Penicillin discovered by Alexander Fleming in 1928 • Observed contamination of bread mold Penicillium inhibited growth of Staphylococcus aureus on agar plate • Could naturally produced molecule kill microbes?

Bacterial ribosomes are an excellent target for selective toxicity.

• They are different in structure from eukaryotic ribosomes.

R0 values are frequently given as a range because R0 values for a particular infectious agent differ depending on age, population density, and environmental conditions that may affect the transmission of the agent, such as temperature and humidity. In general terms, R0 is a function of

(1) density of susceptible contacts (2) transmissibility of the agent, (3) the length of time an individual is infectious. Analyses of virtual and real data from past outbreaks generally confirm that if R0 < 1, the disease will not become an epidemic.

Modification of Preexisting Genes Random mutations form the basis of all genetic changes and occur because of

(1) the imperfect ability of polymerases to precisely copy their template, resulting in spontaneous mutations (2) exposure to mutagens, resulting in induced mutations

The Rapid Rise of Resistance On average, resistance to a new antibacterial drug occurs in just four to five years after a drug's introduction . How does resistance appear so quickly? This is due to the combined action of

(1) transfer of whole resistance genes by mobile genetic elements (2) high frequency of random mutations (3) recombination events (4) short generation times. Consider that most medically important microorganisms can double their population in less than 24 hours, at least in culture.

The Polio Vaccine Compromise As miraculous as attenuated OPV seems, it has a flaw. Attenuated poliovirus replicates, and any virus that replicates, mutates. Rare cases of poliomyelitis sometimes appear following vaccination with OPV. Analysis of the virus strains from these cases confirm they are not wild virus, but genetic deviants of the vaccine strains. The attenuated strains infrequently regain their lost virulence by either recombining their RNA with other related intestinal viruses (type C enteroviruses) of the Picornaviridae family or, less often, by accumulating point mutations. It is estimated that less than 1 in every 2.7 million first-time doses of OPV results in vaccine-associated paralytic poliomyelitis. Because of this small but real risk, the WHO no longer recommends OPV-only vaccination. Out of concern for the risk of vaccine-associated paralytic poliomyelitis, the United States banned the routine use of OPV in 2000, and several other polio-free countries have followed suit. This is in accordance with WHO recommendations for countries where

(1) wild poliovirus has been eliminated, (2) vaccine coverage rate is high (90 to 95 percent), (3) the risk of wild poliovirus importation and transmission is very low.

B-lactams

--> Peptioglycan transpeptidases -cyclic amides -4 membered rings -highly reactive due to large ring strain

• No vaccine is 100%effective,but efficacy has been studied extensively. -Childhood vaccine effectiveness rates are usually 85-90%. -Estimating infected individuals in a population becomes a numbers game.

-1,000 individuals, all vaccinated, exposed to measles virus—how many will fall ill? -Now ramp this number up to a rough estimation of the population of the U.S. (375 million)—How many people might fall ill in a measles epidemic, even if we had 100% of the individuals vaccinated?

Glycopeptides (Vancomycin)

-> Peptidoglycan peptide subunits

Antiviral drugs As we have seen in Chapter 8, viruses use many host cell systems during the production of new viral particles. Because all viruses utilize the host cell translational machinery to make viral proteins, antiviral drugs cannot target this process. Many viruses also use host cell proteins for DNA replication and transcription. For these viruses, antiviral drugs ideally should not target these processes. So what processes can an antiviral drug target? There are two general approaches to antiviral drug design:

-One approach has been the development of broad spectrum antiviral drugs that are effective against several viruses. The other approach is to design specific drugs to target specific viruses. -For this, detailed knowledge of mechanisms for attachment, entry, uncoating, and egress for individual viruses is used. The majority of antiviral drugs available target HIV, herpes viruses, hepatitis B and C viruses, and influenza viruses. The list of antiviral drugs, although not all covered here, is relatively short compared to those available for bacterial infections.

Resistance arises quickly through combined action of

-Transfer of whole resistance genes by mobile genetic elements -High frequency of random mutations -Recombination events -Short generation times (24 hours or less for many microbes versus 2.5 days for some of the fastest-reproducing multicellular organisms)

Polio: A near success

1) Poliovirus is transmitted via the fecal-oral route. • In most cases, a mild gastrointestinal illness results. • In a smaller percentage of cases, the virus "takes a wrong turn" and enters the central nervous system. 2)This can result in varied degrees of paralysis

What are the three ways for inhibition of peptioglycan synthesis?

1)B- lactam--> Peptioglycan transpeptidases 2)Glycopeptides--> Peptidoglycan peptide subunits 3)Bacitrcin ( topical use) --> Isoprenyl pyrophosphate

Antibacterial drugs: Inhibitors of ribosome function

1)Bacterial ribosomes are an excellent target for selective toxicity. 2)Many of these drugs exploit those differences.

Vaccine design: Attenuated vaccines

1)Composed of living (but weakened) pathogen. 2)Tend to produce high immunity because the microbe replicates in the body. -This exposes the immune system to a higher level of foreign antigen over a greater period of time. -An added benefit is the individual might shed vaccine microbe to other individuals, indirectly vaccinating them. 3)There is a possibility that the weakened pathogen could revert to a more pathogenic state, however.

Resistance mechanisms of bacteria have been the most thoroughly studied because bacterial drug resistance is a widespread phenomenon, associated with almost all bacterial pathogens. As shown in Figure 24.10, one or more of the following four mechanisms is generally used to render drugs ineffective:

1)Drug inactivation: Enzymes are produced that destroy or structurally modify the drug. 2)Alteration of drug target site: The drug-binding target is modified. 3)Drug inaccessibility: The cell prevents the drug from entry. 4)Drug efflux: The drug enters, but is quickly exported out of the cell.

Other diseases, such as HIV/AIDS and malaria are of global importance because they affect hundreds of thousands of people every year, yet there is no vaccine. Indeed, several vaccines have been formulated but trials in humans have shown poor efficacy. Why?

1)First, as we have learned, the virus mutates rapidly, so any vaccine targeting immune-protective antigens, such as gp120 will not be effective for long. 2)Second, it can be reasoned that the vaccine target gp120 is absolutely needed for attachment to host cells, so there must be some epitopes of this glycoprotein that cannot be mutated. Although this is indeed the case, vaccines designed to these constant epitopes do not result in antibody that is protective. Recall that vaccines rely on the generation of protective immune responses, and formulating vaccines to simply produce an immune response is often not enough.

The Triangle of Epidemiology A useful conceptual model for controlling infectious disease is the triangle of epidemiology . This framework illustrates the connection between the three major factors associated with the development of an epidemic. These are:

1)Host factors: age, sex, kinship, immigration status, immune status, presence of other diseases, medications. These factors may identify hosts that are most susceptible to the infectious agent. 2)Agent factors: identity of the agent, its habitat, its virulence, incubation period, how it is transmitted or contracted, its mode of entry into the body. 3)Environmental factors: physical aspects of the environment such as changes in climate (temperature, rainfall, drought), sanitation, changes in food or water supply, crowding, location (hospitals, schools). Environmental factors also include cultural and social factors, transportation, and hygienic practices—anything that influences exposure of individuals to the agent.

Many of these drugs exploit those differences.

1)Macrolides bind near the 50S peptidyl transferase site, blocking elongation. 2)Tetracyclines bind to the 16S rRNA portion of the 30S subunit, blocking tRNA anticodon binding

Natural selection and drug resistance Two aspects must be considered:

1)Most treatment works synergistically with the immune system. -It doesn't have to eliminate all microbes -It just has to give the immune system a fighting chance 2)Therapeutic levels must be used that are high enough to help eliminate microbes, but low enough to avoid toxicity • To determine these levels, susceptibility testing is performed A)MIC- minimum inhibitory concentration, the lowest amount of a drug needed to inhibit replication of the microbe B)MLC- minimum lethal concentration, the lowest amount of a drug needed to kill a microbe

If the immune system can clear most infections with the help of antimicrobial drugs, then how do drug-resistant microbes arise?

1)Resistant microbes that remain after the drug course is finished can continue to evade immune defenses and proliferate. 2)The descendants of even partially resistant microbes can acquire more genetic changes, some of which may provide an even higher level of resistance. With further drug exposure and selection, the generational line of microbes goes from susceptible to fully drug resistant. 3_A similar situation occurs when patients are noncompliant and stop taking the drug when they feel better, or miss doses. To treat diseases like malaria and tuberculosis, multiple antimicrobial drugs need to be taken two or three times a day. The average length of treatment for tuberculosis is 6 to 9 months. It is easy to miss a dose or even an entire day during that period and this increases the risk for selection of drug-resistant microbes.

Resistance to Chloramphenicol Chloramphenicol resistance, which is common in many bacterial strains, occurs in two major ways:

1)The production of chloramphenicol acetyltransferase (drug inactivation), 2)The reduction of membrane permeability to the drug (drug inaccessibility).

These three factors will be operating in a concerted balance during an epidemic. Therefore, changing the parameters of any one of the three factors can prevent or stop the spread of an epidemic because it severs a connection in the triangle. Control strategies that target these connections generally involve:

1)The protection of susceptible populations, 2)The containment or elimination of the source of infection, 3)The disruption of transmission.

Antibacterial drugs: Inhibitors of peptidoglycan synthesis

1)These drugs can inhibit crosslinking of PTG or interfere with a step in its actual synthesis. 2)Naturally occurring penicillins bind to penicillin-binding proteins (PBPs) in microbes for transport into the cell. 3)• Some microbes lacking PTG cell wall structures are naturally resistant to these drugs.

Susceptibility testing: Kirby-bauer disk diffusion test

1)Use of antibody- impregnated disks, with a known concentration of antibiotics 2)Drug diffuses into agar at known rate 3)Useful to determine what drugs are most useful for effective therapy

Vaccine design: Inactivated vaccines

1)Vaccine design: Inactivated vaccines • Consist of whole virus/cells that have been inactivated by heat or chemicals. Benefits include that the microbe can't revert, can't replicate, and can't spread. 2)Drawbacks include lower/shorter stimulation of immune responses, a need for multiple injections, and greater risk of negative side effects.

How does a physician determine which drug, what concentration, and how long it should be used in treating an infection?

1)susceptibility testing 2)minimal inhibitory concentration (MIC). In vitro susceptibility testing is done to establish the sensitivity of a particular microorganism to a panel of select antimicrobial drugs and is used to determine which antimicrobial drug will be most successful in treating a bacterial infection in vivo. Susceptibility patterns of various bacteria and fungi are examined by both in vitro and in vivo tests. Prior to drug approval for clinical use, controlled studies are done in vivo to determine how useful a particular antimicrobial drug may be. This includes determining how stable it is inside the body, how quickly it may be metabolized or excreted, and what the effective, therapeutic, nontoxic dose is in patients. This therapeutic dose is related to the minimal inhibitory concentration (MIC). The MIC of a drug is the lowest concentration at which no growth of the microbe being tested occurs in vitro after a standardized period of time. The MIC gives a measure of the drug susceptibility of a microbe.

Investigating an Outbreak A critical public health measure in the prevention of an epidemic is to have accurate surveillance information systems that report the first possible signs of an outbreak. Identifying a localized outbreak can help contain the pathogen and prevent its spread to epidemic proportions. To investigate an outbreak, a systematic procedure is used to identify the agent and the source of infection. These basic "steps" are presented below, and they are often performed simultaneously during outbreak investigations.

1. Define a case. 2. Identify the causative agent. 3. Examine the incidence pattern. 4. Determine the magnitude of the problem. • Attack rate: (number of new cases during time interval/total number of individuals in exposed population) × 100 5. Examine data for common host/environmental factors. 6. Develop a working hypothesis to identify the three points of the triangle of epidemiology. 7. Test the hypothesis by collecting data/performing controlled tests in animal models.

Drug resistance due to antimicrobial drug use Mechanisms vary quite widely, but two include

1. Modification of preexisting genes 2 Acquiring new genes conferring resistance through

The origin of drug resistance: Natural resistance

1.Intrinsic drug resistance due to natural structure or metabolism 2. Resistance genes in the environment

Aminoglycosides and Tetracyclines

16S rRNA of 30S ribosome subunit

Tetracyclines

30s ribosome subunit

Agent factors

: identity of the agent, its habitat, its virulence, incubation period, how it is transmitted or contracted, its mode of entry into the body.

Vaccine design: Subunit vaccines

A)Consist of 1+ isolated protective antigens (no whole cells or viruses) 1)Defined composition is safer. 2)May require several injections to produce strong immunity. 3)The current DTaP (or Tdap) vaccine (diphtheria, tetanus, and pertussis) is an example of this type. B) Conjugate vaccines are a modified form of this method. • They link a polysaccharide antigen to an immunogenic protein. • The idea is that the polysaccharide is a poor antigen on its own. • When you link it to a strong stimulating protein, you can get better responses overall. • The Hib vaccine, protecting against H. influenzae type bacteria that cause meningitis, is an example.

How are epidemics predicted and controlled?

Although antimicrobial drugs are invaluable for treating individuals with infectious disease, they are not used routinely as a public health measure for prevention of infectious diseases, particularly communicable diseases. As we have just learned, excessive use of antimicrobial drugs leads directly to drug resistance, disrupts normal microbiota balances, and can cause toxicity with prolonged use—not to mention the great expense of such long-term communal use. More practical and effective public health measures are employed to prevent those microbial diseases that present a significant public health risk, that is, communicable diseases capable of causing epidemics. These measures include immunization, the topic of Section 24.5, containment of the pathogen, and predicting epidemics in order to avert their occurrence.

Inhibiting ribosome function

Aminoglycosides Macrolides Tetracyclines chloramphenicol

what is an ideal vaccine

An ideal vaccine is one that generates a high level of immunological memory to protective antigens—those antigens that can elicit an immune response that protects against infection and/or disease. Figure 24.17 shows the currently recommended vaccines for routine immunizations in the United States. Table 24.6 provides descriptions of the vaccines. A good vaccine should also not produce serious side effects when used in large populations. There are a number of different ways vaccines are formulated to expose the immune system to protective antigens. Most vaccines follow the four basic designs outlined below.

Antibiotics • Structure plays an important role in antibiotic effectiveness Structure of the drug molecule is critical Structure of the microbe itself is also important

Antibiotics

_________________________ are an important group of antimicrobial drugs and are the most widely prescribed therapeutic drugs. The term antibiotic was originally used to describe compounds produced naturally by bacteria or fungi that inhibit the growth of other bacteria or fungi. However, many naturally produced antibiotics have been modified into semisynthetic compounds by chemical alteration. Many antibiotics are also now produced through cost-effective chemical processes only and are no longer isolated from their associated microorganisms. Synthetic antibiotics do not originate from microbes; they are designed and produced through technology. Because of the widespread use of synthetic and semisynthetic therapeutic compounds against microbial cells, the term "antibiotic" is often used to include all antimicrobial drugs that act on the cells of microorganisms, but not viruses or subviral particles.

Antibiotics

• Antibiotics often interfere with Peptidoglycan synthesis Membrane integrity DNA synthesis Transcription Folic acid synthesis Ribosome function Structure often determines what can be affected.

Antibiotics

Free Radical Formation _____________________is an antimalarial drug, originally isolated from the plant Artemisia annua, but it is also synthesized by genetically engineered yeast. Use of __________________alone gives rise to artemisinin resistance but, when used in combination with other drugs such as mefloquine, remains an effective treatment against chloroquine-resistant strains of Plasmodium falciparum. The mode of action of artemisinin is not completely clear, but is active against the erythrocyte stage of the parasite . ________________ forms reactive free radicals in the presence of ferrous iron (Fe2+), and it appears that the free radicals irreversibly inhibit a metabolic enzyme, the malarial calcium-dependent ATPase, resulting in parasite death.

Artemisinin Artemisinin artemisinin

.____________________ are antibacterial drugs that can kill bacteria directly. Bacteriostatic drugs prevent replication of bacteria, but do not kill them. These descriptions are based on observations of controlled in vitro experiments. A culture treated with a bacteriostatic agent for a short time resumes growth when the agent is removed; the effect is reversible.

Bactericidal drugs

Antibacterial drugs Before we begin to examine particular drugs, it is important to understand how these drugs work in and out of the body to control bacterial growth.____________________ are antibacterial drugs that can kill bacteria directly. Bacteriostatic drugs prevent replication of bacteria, but do not kill them. These descriptions are based on observations of controlled in vitro experiments. A culture treated with a bacteriostatic agent for a short time resumes growth when the agent is removed; the effect is reversible. In a culture treated with a bactericidal agent for a short time, cells are killed; the effect is irreversible. Both bacteriostatic and bactericidal drugs are effective therapies against infection. Inside the complex environment of the body, bacteria are subject to host immune defenses, not just drugs, so any bacterial cells that are unable to grow will usually be destroyed by the immune system within the course of the antimicrobial treatment.

Bactericidal drugs

directly kill the microbes treated

Bacteriocidal

Prevent replication of bacteria but do not kill them

Bacteriostatic

How does the B- lactam rings work?

Beta-lactam antibiotics mimic the terminal alanine amino acids of peptidoglycan precursors, causing the PBPs to bind to them irreversibly. In this state, they can no longer function in the synthesis of peptidoglycan. Different β-lactams have different abilities to interfere with peptidoglycan synthesis. This is related to differences in PBPs and the ability of β-lactams to access them.

__________________ diffuses across plasma and lysosomal membranes of host cells and Plasmodium parasites, which reside inside erythrocytes during a critical stage in their life cycle.________________binds to hemozoin, a nontoxic derivative of heme formed by the parasite as it consumes hemoglobin. Binding prevents crystallization of hemozoin and inhibits the ability of the parasite to continue to detoxify heme inside the acidic food vacuole. Free heme accumulates in the parasite food vacuole and destabilizes the vacuole membrane, resulting in the death of the parasite. The crystallization of heme is a process unique to the parasite, thus drugs that interfere with this process do not significantly harm host cells.

Chloroquine Chloroquine

Resistance Genes in the Environment A second way resistance arises in natural settings is through the development of specific antibiotic resistance genes. Genetic evidence has shown these genes were present long before the use of antibiotics as drugs. How can this be?

Consider that most microorganisms live in complex, highly competitive ecosystems. Antibiotic production in these habitats can secure an advantage by limiting the growth of competitors for resources. Of course, to compete successfully with these antibiotic producers, microorganisms counter with the development of resistance genes. In fact, some resistant bacteria can even use the antibiotics produced by other microorganisms as a nutrient source as evidenced by the isolation of several antibiotic-consuming species from soil. When grown in the presence of clinically relevant concentrations of antibiotics, these bacteria remained unaffected and indeed were able to grow on media containing the antibiotic as a sole carbon source. Several of these isolates are resistant to multiple antibiotics from different classes. In such natural settings, the pressure of microbial competition leads to antibiotics and antibiotic resistance genes.

Inactivation of chloramphenicol by chloramphenicol acetyltransferase (CAT)

Cytoplasmic CAT transfers the acetyl group from acetyl coenzyme A onto chloramphenicol. The presence of the acetyl group prevents chloramphenicol from binding to its ribosome target. Bacterial mutations that increase the expression of CAT produce a high level of resistance to the drug.

Metronidazole

Destruction of DNA target: DNA

Polyenes

Disruption of Plasma membrane target: Plasma Membrane ergosol

Azoles

Disruption of Plasma membrane Target: C1,4 demethalase

Antifungal/ antiprotozoal drugs Much more problematic (eukaryotes!) o Very few available drugs for antifungals, and the few must focus on

Disruption of cell ergosterol (instead of cholesterol in human cell membranes) Inhibition of chitin cell wall structures Selective inhibition of fungal mitosis (difficult!)

Control of infectious disease Human control of disease has made incredible progress in the last century

Due to vaccines,antimicrobial drugs, and improvements in sanitation and hygiene In United States, premature death from infectious disease has dropped considerably

Inhibition of Cell Wall Synthesis ____________________ are a group of originally natural antibiotics produced from a variety of fungal isolates that now have many semisynthetic derivatives. They inhibit synthesis of the major cross-linked glucan found exclusively in fungal cell walls by targeting β-1,3-d-glucan synthase. Their action leads to leakage and lysis and thus they have been called the "penicillins of antifungals." Their inhibitory spectrum includes Candida spp., Aspergillus spp., and Pneumocystis carinii, all common culprits in infections of immunocompromised people, such as those living with HIV/AIDS.

Echinocandins

Inhibition of cell wall synthesis

Echinocandins are a group of originally natural antibiotics produced from a variety of fungal isolates that now have many semisynthetic derivatives. They inhibit synthesis of the major cross-linked glucan found exclusively in fungal cell walls by targeting β-1,3-d-glucan synthase. Their action leads to leakage and lysis and thus they have been called the "penicillins of antifungals." Their inhibitory spectrum includes Candida spp., Aspergillus spp., and Pneumocystis carinii, all common culprits in infections of immunocompromised people, such as those living with HIV/AIDS.

drug inactivation

Enzymes are produced that destroy or structurally modify the drug.

Epidemics involving known infectious agents are rarely completely unpredictable. Even epidemics involving emerging diseases, such as HIV/AIDS, SARS

Even epidemics involving emerging diseases, such as HIV/AIDS, SARS (see Section 5.4), and Ebola, which have only recently entered human populations, did not appear spontaneously (see Sections 18.5 and 22.4). Having studied the viruses responsible for these epidemics, researchers know something about their evolutionary history, how they are transmitted, and who is susceptible. For example, as soon as enough information was gathered in the 1980s regarding HIV, models were developed that predicted HIV would cause significant epidemics in populations practicing unprotected sex with multiple concurrent partners. Unfortunately, this is exactly what happened in many countries. Health care policies and implementation strategies failed to address the aforementioned host and environmental factors needed to stop the epidemic spread of HIV. Where policies have been put into place to address these factors, incidence rates of HIV infection are beginning to stabilize or decrease (see Figure 18.13). For all known communicable diseases, a set of predisposing conditions allows the development of an epidemic in a population. Identifying those parameters can help predict, control, and even prevent epidemics. Let's examine some of these parameters.

So, if a few resistant microbes are expected to exist in every population, doesn't that mean some resistant microbes will always remain in a patient after every antimicrobial treatment? Does every patient become a breeding ground for resistant microbes? Of course, the answer is no, or antimicrobial drugs would be relatively useless. Two important aspects must be considered:

First, the effectiveness of antimicrobial drugs comes from their ability to work in synergy with the immune system. Antimicrobial drugs do not need to destroy every single infecting microbe to be effective. Second, therapeutic concentrations of antimicrobial drugs are sufficiently high to discourage selection for resistance while still being safe for use in vivo. How does a physician determine which drug, what concentration, and how long it should be used in treating an infection? In vitro susceptibility testing is done to establish the sensitivity of a particular microorganism to a panel of select antimicrobial drugs and is used to determine which antimicrobial drug will be most successful in treating a bacterial infection in vivo. Susceptibility patterns of various bacteria and fungi are examined by both in vitro and in vivo tests.

Herd immunity At the level of populations, mass vaccination reduces or prevents epidemics of communicable diseases and can even entirely eliminate some infectious diseases from populations. To accomplish this, does every person need to be vaccinated?

Fortunately, the answer is no. Some individuals will be too remote to contact, and others will have underlying health conditions that make vaccination inadvisable. This includes individuals who are immunocompromised, those who have had allergic reactions to previous vaccinations, or those with a history of autoimmune disease. Some people refuse to be vaccinated because of religious choice, or philosophical reasons, as pointed out in Perspective 24.4. Lastly, as we have seen, a proportion of vaccinated individuals will experience vaccine failure. Fortunately, a population effect known as "herd immunity" can protect non-immune individuals and stop transmission of a communicable disease.

Artemisinin

Free Radical formation target: marlarial calcium dependent ATPase

Antimicrobial Resistance in Farmed Animals The extensive use of antimicrobial drugs over time in human medicine has without doubt led to the rapid rise and spread of resistant microbes. However, agriculture is the largest consumer of antimicrobial drugs. Estimates vary, even within countries:

From 40 to 80 percent of all antibiotic use is in farmed animals. What is strikingly different from human applications is how and why these antibiotics are used in animals. The majority of the antimicrobial drugs used in farmed animals are not used to treat infections, but to promote growth of the animals. Massive amounts of data confirm cattle, swine, and poultry fed with subtherapeutic levels of antimicrobial agents show superior weight gain and improved feed efficiencies compared to their antimicrobial-free counterparts. For example, pigs show an increase in weight from 3.3 to 8.8 percent and feed efficiency improves from 2.5 to 7 percent, meaning less food needs to be fed to these animals to achieve an equivalent weight gain to their counterparts not receiving antimicrobial drugs. Similar data exist for cattle and poultry. How low levels of antimicrobial drugs promote growth is not precisely known. Most researchers think that the effects are multifactorial. Studies support evidence for suppression of infectious diseases, such as dysentery, and alteration of normal commensal microbial communities of the gut. It is estimated that as much as 6 percent of the energy in a pig's diet may be harvested by the gut microbiota, instead of by the pig. Reducing microbial population sizes through antimicrobial use may lead to more nutrient uptake for the animal.

Bacitrcin

Gram + Isoprenyl pyrophosphate

In_________________________, PBPs are found in the membrane-bound periplasm, whereas in __________________, they are found within the external cell wall . Natural penicillins G and V are not transported across the outer membrane of____________________, and therefore are only effective against ___________________. However, the addition of amino and carboxyl groups to the penicillin side chain allows these semisynthetic penicillins to be transported through outer membrane porins to act on the periplasmic PBPs.

Gram-negative bacteria Gram-positive bacteria Gram-negative bacteria Gram-positive bacteria

Disruption of Mitosis ___________________is an oral antibiotic used against a few fungal genera causing skin (commonly, ringworm caused by Microsporum spp.), hair, and nail infections. The drug binds keratin precursors and becomes incorporated into new keratin, which delivers the drug to infecting fungal cells. The fungal cells transport it in where it binds tubulin, inhibiting the formation of microtubules needed for mitosis.

Griseofulvin

Anti-fungal drugs

Griseofulvin Echinocandins Polyenes Azoles

Disruption of Mitosis

Griseofulvin is an oral antibiotic used against a few fungal genera causing skin (commonly, ringworm caused by Microsporum spp.), hair, and nail infections. The drug binds keratin precursors and becomes incorporated into new keratin, which delivers the drug to infecting fungal cells. The fungal cells transport it in where it binds tubulin, inhibiting the formation of microtubules needed for mitosis.

Historic aspects of infectious disease The ancient Greeks were the first to separate religious superstitions from disease

Hippocrates (circa 400 BC) lectured that disease was a result of physiological imbalance He promoted the idea of restoring balance through diet Others took his ideas to extremes, using bloodletting or leeches to try to remove "excess" blood to "restore balance"

acquisition of new genes

Horizontal gene transfer events involve large-scale sequence changes that are brought about by mobile genetic elements, such as plasmids, transposable elements, and transducing phages, that facilitate horizontal gene transfer. Indeed, these elements can transfer entire resistance genes very quickly from one microbial cell to another. You will recall from Chapter 21 that horizontal gene transfer also moves virulence genes around in a rather promiscuous manner in bacteria, leading to gene transfer in related as well as unrelated species. This happens for resistance genes as well.

_______________________ interferes with translation and thus protein synthesis in the bacterial cell. ____________________ such as kanamycin, neomycin, gentamicin, and their derivatives, act on the ribosome by reversibly binding to 16S rRNA, disrupting interaction of mRNA with tRNA, thus interfering with translation. ______________________________are known for their toxicity in humans and animals, particularly to kidneys and the inner ear apparatus. The mechanism of toxicity remains unknown, but is unrelated to protein synthesis in eukarya. Lower doses are less toxic, but not as effective. For this reason, they are frequently used in combination with other antibacterial drugs.

Inhibiting ribosome function Aminoglycosides, Aminoglycosides,

Intrinsic drug resistance

Intrinsic Drug Resistance Many microbes are resistant to drugs by virtue of their natural structure or metabolism. They do not possess specific resistance genes to target antimicrobial drugs. We have already seen that Gram-negative bacteria are naturally resistant to some antibacterial agents because the drugs simply cannot pass through the outer membrane. The wall-less bacterial genus Mycoplasma is naturally resistant to β-lactams because members do not produce peptidoglycan. The thick layer of hydrophobic mycolic acids that characterizes members of the genus Mycobacterium similarly prevents water-soluble drugs from accessing the peptidoglycan or the plasma membrane and prevents transport into the cell. In addition, pathogenic mycobacteria produce intracellular infections, so any drug that is active against them needs to enter both the host cell and the bacterium in order to be effective.

Foot-and-mouth disease: The making of an epidemic Most of the examples we have been discussing have been human communicable diseases. How are communicable diseases of animals controlled?

Let's examine foot-and-mouth disease virus (FMDV), a close relative of poliovirus that has significant impacts on domestic herds

Antiprotozoal drugs

Like fungal infections, combating protozoal infections is difficult. Not only do protozoa share many structural and metabolic features with the cells of their hosts, but also they share little with each other. Consequently, relatively few broad spectrum and selectively toxic drugs are available, and the toxicity associated with these commonly causes side effects such as vomiting, diarrhea, and nausea, further complicating this is the ability of many protozoa to differentiate into distinct forms through their life cycle, and to migrate to different tissues. As a result, many protozoa present a moving target, and treatment of these pathogens can be complicated, requiring a regimen of different drugs. Here, we will look at only a few common drug groups, focusing on antimalarial drugs because they form the largest group of antiprotozoal drugs.

The origin of drug resistance genes Where do drug resistance genes originate?

Logically, if natural selection is working at the genetic level, then the genes for microbial resistance must have been there in some form before exposure to the drugs. Let's examine where these genes may have originated.

Altered PBPs The most notorious example of PBP-mediated resistance is that of methicillin-resistant Staphylococcus aureus, commonly referred to as ___________________. Staphylococcus aureus normally has genes coding for five different PBPs, all highly susceptible to inactivation by β-lactams. However, MRSA strains have acquired a mobile genetic element from an unknown source, carrying a sixth PBP gene. At least two genes, known as mecA and mecC have been discovered to code for this newly acquired PBP. These new PBPs are resistant to β-lactams, yet have sufficient peptidyl transferase activity to provide peptidoglycan synthesis in the absence of activity from the other five. It is interesting to note that although methicillin is no longer used clinically, having been replaced by oxacillin and cefoxitin, the label "MRSA" continues to be used as it represents resistance to all β-lactams.

MRSA

___________________act on the bacterial ribosome by binding near the 50S peptidyl transferase site, blocking elongation of the growing peptide. Like __________________they inhibit protein synthesis. Figure 24.6 summarizes the action of many ribosome-inhibiting antibiotics. The macrolide erythromycin is effective against Gram-positive bacteria, but is not efficiently taken up by most Gram-negative bacteria.

Macrolides aminoglycosides, tetracyclines, and chloramphenicol,

Destruction of DNA _____________________, commonly known by its tradename, Flagyl, is a synthetic drug active against some protozoa and anaerobic bacteria that take up the drug and then convert it to its active form. The active form is incorporated into the DNA helix, causing lethal breakages. Flagyl is used to treat intestinal infections of Giardia, Entamoebae, and the anaerobic Clostridium difficile bacteria as well as Trichomonas vaginalis, a sexually transmitted protozoan.

Metronidazole

nucleoside analogs

Mimic structures of purines (A, G) and pyrimidines (C, T)

antiviral drugs

Necleoside analogs Amantadine Neuramindase inhibitor

Natural Resistance

Not all microbes become resistant to antimicrobial drugs through the use of these drugs. Indeed, drug resistance is not reserved for pathogens. Many environmental microbes that are not involved in infectious disease also exhibit drug resistance when tested. Although this may seem surprising, this natural resistance has arisen through evolution to accommodate selective pressures unrelated to drug use. Let's examine some of the ways natural resistance occurs and relate this to antimicrobial drugs.

Disruption of Plasma Membrane Desired selective toxicity for an antifungal drug requires a target feature unique to fungi. What would represent such a target

One distinguishing feature of fungi is the presence of ergosterol, instead of cholesterol, in their plasma membranes. Polyene antibiotics bind ergosterol to form pores in the plasma membrane, causing increased membrane permeability, leading to detrimental ion imbalances. Azoles comprise a major group of antifungal drugs and are also widely used for treating and preventing fungal infections in agricultural crops. They disrupt membrane sterol synthesis, including ergosterol synthesis, leading to membrane damage. Some are used systemically, but most are topical. Azoles, such as fluconazole, are commonly used for Candida vaginal and skin infections.

Resistance to Sulfonamides Recall from Figure 24.8 that sulfonamides interfere with folic acid synthesis, needed for nucleic acid synthesis, by competing with ____________________for the enzyme dihydropteroate synthetase. The major mode of resistance to sulfonamides is through alterations of dihydropteroate synthetase (alteration of drug target site). By producing a modified dihydropteroate synthetase enzyme that has reduced affinity for the drug, folic acid can be synthesized in sufficient amounts. Some bacteria can increase their ability to take up preformed folic acid from their environment, rendering the metabolic pathway for folic acid redundant. Similarly, resistance for the drug trimethoprim, which interferes with the same metabolic pathway, occurs by modification of the enzyme dihydrofolate reductase

PABA

Most patients recover from this, but in a few, the motor neurons become infected, resulting in nerve damage and paralytic poliomyelitis. ________________ results in partial or complete paralysis and is estimated to occur in less than 1 percent of those who develop poliomyelitis. A proportion of paralytic poliomyelitis victims may die due to respiratory failure involving paralysis of the diaphragm.

Paralytic poliomyelitis

Drugs Tailored to Specific Viruses Our understanding of viral replication is directly linked to our ability to develop effective antiviral drugs. The initial binding of the virus to a host cell can be targeted.________________of the plasma membrane could be targeted. For viruses that replicate in the host cell nucleus, movement through the cytoplasm to the nucleus could be targeted. Even aspects of assembly and exit from the cell could be targeted. However, the options are somewhat limited because rational design and development of tailor-made drugs is only possible if one has a detailed understanding of how viruses replicate.

Penetration

Macrolides

Peptidyl transferase of site 50S ribosome subunit

However, the virus becomes systemic in some cases, entering the circulatory system and gaining access to the central nervous system. ________________________ then develops as a result of inflammation of the meninges, the layer of tissue surrounding the brain. Most patients recover from this, but in a few, the motor neurons become infected, resulting in nerve damage and paralytic poliomyelitis.

Poliomyelitis

Polio Disease and Transmission

Poliovirus is a very small non-enveloped RNA virus transmitted via the fecal-oral route. Humans ingest the virus from contaminated water and food or through contact with feces-contaminated objects. The virus attaches to a specific immunoglobulin-like receptor known as the poliovirus receptor, which is present on epithelial cells lining the pharynx and small intestine and also on leukocytes and motor neurons. Viral replication in epithelial cells of the digestive tract results in shedding of large numbers of infectious virions. In areas where poliovirus is still endemic , it can be found in lakes, rivers, and streams contaminated by raw human sewage.

Antiprotozoan drugs

Quinoline Artemininin Metronidazole

Inhibition of Heme Detoxification __________________not to be confused with antibacterial_______________,are antimalarial agents related to quinine, a compound extracted from the cinchona tree of Peru and Bolivia, but can also be made synthetically._________________was commonly used to treat malaria as long ago as the seventeenth century, but it is highly toxic. In the 1940s, it was replaced by related but less toxic synthetic drugs such as chloroquine. Resistance to quinine is unusual, and it is still used in cases in which the disease-causing agent is resistant to these newer drugs.

Quinolines quinolones, Quinine

___________________ are two of the most commonly used synthetic antibacterial drugs. Quinolones are based on the core structure of nalidixic acid, one of the first quinolones to be used. Newer versions are fluorinated to give them better antibacterial activity and less toxicity. Quinolones are broad spectrum drugs that bind to DNA gyrase, particularly of Gram-negative bacteria, and topoisomerase IV of Gram-positive bacteria . They can also be used against intracellular pathogens such as Legionella pneumophila, Mycoplasma pneumoniae, and Mycobacterium tuberculosis, because of their ability to enter eukaryal cells.

Quinolones and sulfonamides

Models for propagated epidemics caused by communicable diseases that are spread directly or indirectly between individuals focus on determining the value R0, the basic reproduction number:

R0 is defined as the average number of new infections caused by an infected individual in a population of susceptible individuals. For example, the R0 value for measles is 12 to 18, meaning that an infected person will spread the infection to between 12 and 18 other people on average. R0 values are frequently given as a range because R0 values for a particular infectious agent differ depending on age, population density, and environmental conditions that may affect the transmission of the agent, such as temperature and humidity.

R0 = 1, the epidemic threshold Epidemiologists have formulated various mathematical equations to model and predict epidemics.

R0 is the basic reproduction number: 1)The average number of new infections caused by an infected individual in a population of susceptible individuals. 2)The higher the number, the more quickly the epidemic can spread. 3)The value can vary from population to population depending on age, population density, environmental conditions, transmissibility, etc.

Resistance tends to appear only 4 to 5 years after a drug's introduction. How does it appear so quickly?

Resistance arises quickly through combined action of Drug makers are fighting these problems with new technology for smarter and faster drug design.

Historic aspects of infectious disease • Lack of scientific understanding led to religion and superstition "explaining" diseases

Sickness was a punishment from supernatural beings for immorality or sinfulness Afflicted individuals were often cast out of society

_______________________are very small antibacterial drugs that are structural analogs of para-aminobenzoic acid (PABA). PABA is used by many bacteria to make folic acid, a necessary precursor for the synthesis of the nitrogenous bases adenine, guanine, and thymidine, needed for DNA synthesis. When sulfonamides competitively bind the enzyme dihydropteroate synthetase that incorporates PABA into the folic acid precursor molecule, enzyme function is inhibited and also a non-functional folic acid derivative results. The cells of animals and humans are unaffected by sulfonamide drugs because they take up folic acid, also known as vitamin B9, from their diet. Similarly, many single-celled eukarya and some bacteria acquire folic acid from their environment and are unaffected by these drugs.

Sulfonamides (sulfa drugs)

____________________and their derivatives act on the 30S ribosome by binding to the 16S rRNA, blocking interaction of the tRNA anticodon to the mRNA codon, thereby inhibiting bacterial protein synthesis. They have an inherent ability to bind RNA secondary structures, making them somewhat active against mRNA of eukaryal cells as well. Fortunately, ________________ are not as efficiently taken up by eukaryal cells, but they can cause toxicity in high doses. __________________can permanently discolor teeth, especially in children, due to formation of localized calcium/tetracycline complexes . Tetracycline is effective against many Gram-negatives, most Gram-positives, both of the developmental forms of Chlamydia, and the peptidoglycan-deficient bacteria Rickettsia giving it one of the broadest spectrums of any antibacterial drug

Tetracyclines Tetracyclines Tetracyclines

Resistance (R) plasmids

The R plasmids encode genes that confer the ability to resist the antimicrobial activity of one or often several drugs and are usually transferred from cell to cell by conjugation. From Section 2.4, you may recall that one way to combat the activity of β-lactamases is to combine β-lactams with clavulanic acid. Clavulanic acid binds to β-lactamases and inhibits their ability to cleave the β-lactam ring, thus rendering β-lactam treatment effective. R plasmids have become common in populations of Salmonella Typhi, Escherichia coli, Klebsiella pneumoniae, and Proteus vulgaris.

Drug inaccessibility

The cell prevents the drug from entry.

drug efflux

The drug enters, but is quickly exported out of the cell.

Alteration of drug target site

The drug-binding target is modified.

. What is the evidence for selection for resistance in clinical settings? The following documented global trends provide compelling evidence that antimicrobial drug use is linked to development of resistance:

The following documented global trends provide compelling evidence that antimicrobial drug use is linked to development of resistance: 1)Changes in antimicrobial drug use are positively correlated with changes in prevalence of resistance. 2)Increasing the length of antibiotic treatment in patients is associated with increasing rates of colonization of these patients with resistant organisms. 3)Microbial resistance is highest in facilities where antimicrobial drugs are used with the highest frequency. 4)Patients with resistant strains have received antibiotics more often than patients with non-resistant strains.

So far, we have learned that the herd immunity threshold gives an estimate of the proportion of people that must be immunized. That is, the proportion of people that must gain protective immunity. We have also learned that, due to vaccine failure, not everyone who is vaccinated gains immunity. The practical question that must follow this then, is what proportion need to be vaccinated?

The herd immunity threshold allows estimation of the vaccine coverage rate, the proportion of individuals in the population that must be vaccinated to achieve herd immunity. Usually vaccine coverage target rates are set to slightly exceed herd immunity thresholds, to account for the usually low rates of vaccine failure. For example, the WHO sets vaccine coverage target rates at 95 percent for measles, which has a maximum herd immunity threshold of 94 percent

If the immune system can clear most infections with the help of antimicrobial drugs, then how do drug-resistant microbes arise?

The immune system is not always effective in clearing resistant microbes, and some patients do not comply with taking their antimicrobial drugs. These are the two most common ways resistant microbial populations develop. Figure 24.13 depicts events in an immunocompromised person. 1)Resistant microbes that remain after the drug course is finished can continue to evade immune defenses and proliferate. The descendants of even partially resistant microbes can acquire more genetic changes, some of which may provide an even higher level of resistance. With further drug exposure and selection, the generational line of microbes goes from susceptible to fully drug resistant. A similar situation occurs when patients are noncompliant and stop taking the drug when they feel better, or miss doses. To treat diseases like malaria and tuberculosis, multiple antimicrobial drugs need to be taken two or three times a day. The average length of treatment for tuberculosis is 6 to 9 months. It is easy to miss a dose or even an entire day during that period and this increases the risk for selection of drug-resistant microbes.

Resistance to β-Lactams Resistance to β-lactams is achieved in four major ways:

The production of β-lactamase (drug inactivation). The alteration of penicillin-binding proteins (PBPs) (alteration of drug target site). The reduction of membrane permeability to the drug (drug inaccessibility). The removal of the drug from the cell (drug efflux).

Principles of epidemic control How are epidemics controlled or prevented?

To answer this question, we need to know the parameters that influence the development of an epidemic. These same parameters will be involved in stopping it.

___________________ inhibits another enzyme, dihydrofolate reductase, later in the folic acid pathway. Inhibition of this enzyme prevents synthesis of the folic acid derivative__________________, preventing synthesis of thymidine . ______________________are often used together in a drug combination referred to as "co-trimoxazole."

Trimethoprim tetrahydrofolic acid Sulfonamides and trimethoprim

Inhibitors of nucleic acid synthesis

Trimethoprim Quinolones Sulfonamides

Susceptibility testing: Epsilometer test (Etest)

Use of antibody- impregnated strips, which contain a gradient of antibiotic concentrations. MIC value is read directly off the strip

Vaccine efficacy Are vaccines really effective? Despite the development of many highly effective vaccines, no vaccine is 100 percent protective for every individual. ____________ or the effectiveness of a vaccine, is an average measure of the degree of protection it affords a population. Most routine childhood vaccines have an efficacy range of 85 to 90 percent, meaning between 85 and 90 percent of those given the vaccine will be protected from the associated disease. For largely unknown reasons, a certain proportion of individuals will fail to establish a high enough level of immunological memory to protect them even when they have been vaccinated. This is known as ______________________, although "immune failure" is likely the underlying reason. Ali Maow Maalin, the survivor of the world's last natural case of smallpox, was himself vaccinated before he contracted the disease

Vaccine efficacy, vaccine failure

antibiotic

a medicine used to save lives because it destroys harmful bacteria and cures infections

Foot-and-Mouth Disease Transmission FMDV can spread by every possible means:

aerosol, water, contact, soil, bedding, urine, milk, and saliva. Winds can transmit the virus over many kilometers, and, with an astounding R0 value between 36 and 73, it spreads explosively in unprotected herds. FMDV causes high mortality in infected young animals and severely cripples adult animals by producing painful lesions on feet and in the mouth

Host factors

age, sex, kinship, immigration status, immune status, presence of other diseases, medications. These factors may identify hosts that are most susceptible to the infectious agent

bacteriostatic agent

an agent that inhibits the growth of bacteria

The term ___________________was originally used to describe compounds produced naturally by bacteria or fungi that inhibit the growth of other bacteria or fungi. However, many naturally produced antibiotics have been modified into semisynthetic compounds by chemical alteration.

antibiotic

What kinds of drugs are used to treat infections, and how do they work?

antimicrobial agents: are compounds used to treat or prevent infections primarily of the body. They are a subset of antimicrobial agents, introduced in which include any substance that inhibits or kills microbes, including viruses. In this chapter, we will examine the mode of action of some of the more common antimicrobial drugs.

Attenuated vaccines are

are composed of a live but weakened version of a microbe that is completely or nearly devoid of pathogenicity. These vaccines tend to produce high levels of immunity because the attenuated microbe can replicate to some extent in the body, exposing the immune system to its antigens over a period of time, which results in a strong memory response. Consequently, fewer doses or "boosters" are usually needed. Some attenuated vaccines can be shed from the body and can be passed to other individuals. This contact spread is both a benefit and a disadvantage for attenuated vaccines; it can help protect others but can be transmitted to susceptible individuals in some cases. For example, live vaccinia virus vaccine can be transmitted from vaccinated persons to newborns, where it causes serious disease. Attenuated microbes may also undergo genetic changes, making them more virulent. This has occurred for an attenuated oral polio vaccine formulation, which we will examine near the end of the chapter.

Vaccines

are preparations of microbes or their components that are used in immunization.

When a person ingests poliovirus, a number of outcomes are possible. In most cases, dissemination of poliovirus beyond the epithelial lining of the digestive tract does not reach a significant level. These infections are either _______________, producing mild fever, nausea, vomiting, sore throat, constipation, cramps, and stiffness. Recovery is usually complete.

asymptomatic or short-lived

Determine the magnitude of the problem. Begin by comparing the current incidence with that of past or expected incidence. Then compute the attack rate. The____________is an incidence rate calculated during an outbreak, and gives the proportion of ill individuals in an exposed population over a short period of time. The attack rate is a measure of the probability of an exposed individual becoming a case, and it is used to estimate how many more individuals are expected to become ill.

attack rate

Intrinsic drug resistance

bacteria lacks the target for the drug

A culture treated with a ___________________for a short time resumes growth when the agent is removed; the effect is reversible. In a culture treated with a bactericidal agent for a short time, cells are killed; the effect is irreversible.

bacteriostatic agent

Both______________________ are effective therapies against infection. Inside the complex environment of the body, bacteria are subject to host immune defenses, not just drugs, so any bacterial cells that are unable to grow will usually be destroyed by the immune system within the course of the antimicrobial treatment.

bacteriostatic and bactericidal drugs

How does Tamiflu work?

by preventing the activity of the enzyme that helps a new virus detach from the infected cell

DNA vaccines

consist of a cloned gene(s) in a DNA vector, which is then delivered to cells of the body, usually by injection of an engineered virus, or by electroporation of an engineered plasmid using electrical pulses to create pores in cells of muscle or skin. Expression of the cloned gene produces a protective protein antigen that induces an immune response. In this respect, DNA vaccines are similar to recombinant subunit vaccines. However, because the antigen is expressed in cells over a period of time, DNA vaccines evoke a strong memory response, like that of attenuated vaccines. These vaccines do not contain live microbes and are not able to be passed from host to host. DNA vaccines are largely still in experimental stages, but many appear to be very promising. There are only two currently licensed DNA vaccines. One is for farmed salmon to protect against viral infectious hematopoietic necrosis, and the other is for horses to protect against West Nile virus encephalitis. A counterpart of this vaccine is undergoing clinical trials for use in humans.

Inactivated killed vaccines

consist of whole cells or viruses that have been physically inactivated by heat or chemically inactivated by treatment with a denaturing agent such as formalin. The mixture of antigens that are liberated by the inactivation process is then injected into the body. Properly inactivated vaccines cannot replicate in the body, cannot be spread, and cannot regain virulence. Inactivated vaccines typically require several injections to evoke long-term protective immunity because the immune system is exposed to the antigens only for a short period of time. They also tend to produce a higher level of side effects owing to the complex mixture of components that are injected. Example: An example is pertussis vaccine. The first generation of pertussis vaccine consisted of killed whole cell preparations. It was an effective vaccine, but because it was a crude preparation of microbial components, convulsions occurred in 0.1 percent of infants given the vaccine, and a smaller percentage of these suffered brain damage. The new generation of pertussis vaccines consists of acellullar subunit preparations, described next.

Many antibiotics are also now produced through______________________ processes only and are no longer isolated from their associated microorganisms._____________________do not originate from microbes; they are designed and produced through technology. Because of the widespread use of synthetic and semisynthetic therapeutic compounds against microbial cells, the term _________________" is often used to include all antimicrobial drugs that act on the cells of microorganisms, but not viruses or subviral particles.

cost-effective chemical Synthetic antibiotics "antibiotic

Data from various countries lack reproducibility and comparability, making it difficult to link the use of antimicrobial agents in farming to resistance in human pathogens. Despite numerous criticisms, it has been established in several cases that drug-resistant bacteria originating from livestock have been transferred to humans, and bacteria that have developed resistance to one antibiotic used in feed have also developed resistance to related antibiotics used in humans. This is known as "_______________" and may have grave consequences in the battle against multidrug-resistant microbes. An example is the glycopeptide antibiotic avoparcin, which is not used in human medicine but is widely used as an additive in poultry feed

cross-resistance

Drug Removal Microorganisms can use ___________________ as shown in Figure 24.10, to move a drug directly back out of the cell before it can reach its target. Efflux pumps are active transport protein complexes used to move compounds out of the cell. These systems can transport the drug back across the membrane as quickly as it comes in. Multi-efflux pump systems, frequently associated with multidrug resistance, are promiscuous and can transport a variety of structurally unrelated compounds, including antimicrobial agents, out of the cell. Some clinical strains of Pseudomonas aeruginosa possess an operon containing genes for multi-efflux pumps, capable of efficiently transporting not just β-lactams, but a variety of drugs out of the cell.

efflux pumps

Fuzeon

enfuvirtide Antiviral/HIV/AIDS

Sulfonamides and Trimethoprim

folic acid synthesis inhibitors

What number of immunized individuals are needed for herd immunity? The critical proportion of the population required to be immunized to achieve herd immunity is called the _________________. Table 24.7 shows herd immunity thresholds for some communicable diseases that are controlled through vaccination.

herd immunity threshold

attack rate formula

ill/(ill+well) x 100

herd immunity

immunity in most of the population; outbreaks are sporadic due to the lack of susceptible individuals

Quinolones

inhibit DNA gyrase

β-lactams

inhibit peptidoglycan/cell wall synthesis by inactivating transpeptidases (hydrolysation of D-alanine), only active in GROWING cells (do not combine with inhibitor of protein synthesis)

Necleoside analogs

inhibition of DNA synthesis Target: DNA

Echinocandins

inhibition of cell wall synthesis

Neuramindase inhibitors

inhibition of influenza virus release target: Neuraminide

Amantadine

inhibition of influenza virus uncoating target: influenza

Griseofulvin

inhibition of mitosis Target Tubulin

immunization

is the process of generating immunological memory to prevent infection and/or disease through the introduction of an antigen into the body

Bactericidal drugs

kill bacteria

• Human diseases usually require_________________ of susceptible hosts. • They also require the hosts to stay put long enough to contaminate their water and attract animal vectors (rats, fleas, etc.). • As populations rose in number, and humans ___________________(away from hunting and gathering), diseases became more prevalent.

large populations moved toward agriculture

The drugs ______________________, mentioned in Sections 8.1 and 8.2, are other tailor-designed drugs that act to block HIV entry. ______________ blocks viral attachment by binding to the CCR5 cellular co-receptor and Fuzeon, as its name suggests, blocks fusion between the HIV envelope and the cell plasma membrane Another approach for tailoring drugs to block viral entry is based on monoclonal antibodies.

maraviroc and Fuzeon Maraviroc Fuzeon,

Susceptibility of microorganisms to drugs that act on cytoplasmic or periplasmic targets is a function of the transport of these drugs across membranes. Without transport, they are ineffective. In Gram-negative bacteria, PBPs are found in the _____________________, whereas in Gram-positive bacteria, they are found within the external cell wall . Natural penicillins G and V are not transported across the outer membrane of Gram-negative bacteria, and therefore are only effective against Gram-positive bacteria . However, the addition of amino and carboxyl groups to the penicillin side chain allows these semisynthetic penicillins to be transported through outer membrane porins to act on the periplasmic PBPs. This imparts a much broader range of activity..

membrane-bound periplasm

This includes determining how stable it is inside the body, how quickly it may be metabolized or excreted, and what the effective, therapeutic, nontoxic dose is in patients. This therapeutic dose is related to the _________________________. The MIC of a drug is the lowest concentration at which no growth of the microbe being tested occurs in vitro after a standardized period of time. The MIC gives a measure of the drug susceptibility of a microbe.

minimal inhibitory concentration (MIC)

MIC-

minimum inhibitory concentration, the lowest amount of a drug needed to inhibit replication of the microbe

MLC

minimum lethal concentration, the lowest amount of a drug needed to kill a microbe

With all this gene movement occurring between species, it is not hard to envision how antibiotic resistance genes from environmental sources, such as the soil bacteria mentioned previously, can inadvertently move into microbes carried by humans and animals, and become selected for by antibiotic use. These environmental strains are suspected of being a hidden reservoir for present and future resistance genes. Another source of resistance genes is____________________. Any antimicrobial drug that is used systemically (taken orally or by injection), acts not only on the target microbe, but also on a variety of microbes at various sites, such as the intestinal tract, increasing the possibility of selection for resistance genes in a variety of microbes.

normal host microbiota

Recombination events play one of the most important roles in gene transfer and gene rearrangements. Previously, we defined recombination as the breaking and rejoining of two different _________________ . Within cells, recombination can occur between mobile genetic elements and the chromosome, as well as among the mobile genetic elements themselves. All these recombination opportunities mean that it is a relatively easy process to move a gene from one cell to another; microbes have incredible genetic dexterity.

nucleic acid molecules

Inhibition of DNA Synthesis In the search for broad spectrum antiviral drugs exhibiting selective toxicity, researchers have taken advantage of the fact that, first, many viruses have rapid genome replication rates compared to cellular genome replication rates, and second, many viruses use their own viral DNA or RNA polymerases for replication. Recall from Chapter 8 that single-stranded RNA viruses use a viral RNA-dependent RNA polymerase. Retroviruses use a viral RNA-dependent DNA polymerase. Herpesviruses and poxviruses, which contain double-stranded DNA genomes, use their own DNA-dependent DNA polymerases. This distinction from host cell polymerases has led to the development of a group of antiviral drugs known as_______________________ which competitively inhibit DNA synthesis by becoming incorporated into the growing chain during viral replication. Nucleoside analogs are structurally similar to nucleosides and, once phosphorylated in the cell, compete with normal nucleotide triphosphates for viral DNA polymerases. Azidothymidine (AZT), also called zidovudine, is a therapeutic drug for HIV infection . It is a dideoxythymidine analog that causes DNA chain termination because it lacks the 3´—OH group needed for nucleotide addition. Instead, AZT contains an azide group (—N3) in this position. Because a free 3´—OH group is necessary for DNA elongation , no additional nucleotides can be added to AZT if it becomes incorporated into a growing DNA chain. Thus, this drug prevents the completion of a DNA copy needed for insertion into the host genome for viral replication (see Figure 8.20). The similar chain terminating drug tenofovir, an analog of adenosine, acts to inhibit the reverse transcriptase of HIV and hepatitis B virus. Retroviruses are not the only viruses to use a reverse transcriptase. Hepatitis B virus belongs to family Hepadnaviridae (see Figure 5.26), a family of DNA viruses that use a reverse transcriptase for replication, as can be seen in Figure 8.13.

nucleoside analogs,

Vaccine efficacy

or the effectiveness of a vaccine, is an average measure of the degree of protection it affords a population. Most routine childhood vaccines have an efficacy range of 85 to 90 percent, meaning between 85 and 90 percent of those given the vaccine will be protected from the associated disease.

environmental factors

physical aspects of the environment such as changes in climate (temperature, rainfall, drought), sanitation, changes in food or water supply, crowding, location (hospitals, schools). Environmental factors also include cultural and social factors, transportation, and hygienic practices—anything that influences exposure of individuals to the agent.

Drug resistance genes in clinical strains of microbes come from two sources:

preexisting genes newly acquired genes

Resistance Due to Antimicrobial Drug Use Medically important microbes become drug resistant as a direct consequence of antimicrobial drug use. Even Alexander Fleming noted the appearance of penicillin-resistant Staphylococcus aureus in cultures he was working with in the lab. Penicillin-resistant strains of S. aureus started appearing in patients receiving the drug shortly after it entered widespread use (see Figure 24.2). Drug resistance genes in clinical strains of microbes come from two sources:

preexisting genes and newly acquired genes.

An ideal vaccine is one that generates a high level of immunological memory to ____________________________—those antigens that can elicit an immune response that protects against infection and/or disease.shows the currently recommended vaccines for routine immunizations in the United States. provides descriptions of the vaccines. A good vaccine should also not produce serious side effects when used in large population

protective antigens

β-Lactamase Many different types of β-lactamases exist among bacteria, with some more active against certain β-lactam antibiotics than others. All β-lactamases sever the C-N bond of the β-lactam ring (see Figure 2.19), rendering the drugs unable to bind to their PBP targets. Thus, peptidoglycan synthesis is not disrupted. Beta-lactam-mediated resistance in Gram-negative bacteria is most commonly due to possession of__________________genes for β-lactamases

resistance (R) plasmids carrying

Folic acid deficiency

results in macrocytic anemia

Characteristics of antimicrobial drugs They are often administered internally They exhibit __________________(more toxic to an infectious microbe than to the host/host cells) They may be of natural origin or chemically synthesized They may be highly effective at eliminating one class of microbes (e.g., bacteria) while ineffective at eliminating others

selective toxicity

The reason for this lies in their ability to target specific components of microbes. Drugs that do not exhibit sufficient _________________ are not used clinically. Despite exhibiting selective toxicity, some antimicrobial agents still have the potential to cause damage to the host, usually because their mode of action has some activity on host cells as well.

selective toxicity

ll antimicrobial drugs used for treatment possess______________ they are more toxic to an infecting microbe than they are to the host when used at recommended dosages. The reason for this lies in their ability to target specific components of microbes. Drugs that do not exhibit sufficient selective toxicity are not used clinically. Despite exhibiting selective toxicity, some antimicrobial agents still have the potential to cause damage to the host, usually because their mode of action has some activity on host cells as well. When correctly used at therapeutic doses, these toxic side effects are minimized while maintaining useful antimicrobial activity. Many antimicrobial drugs are never approved for use because they are too toxic to the body at concentrations needed to combat infection.

selective toxicity;

herd immunity

sometimes known as "community immunity," occurs when successful immunization of a certain proportion of a population serves to protect the rest of the population from communicable disease. A lack of susceptible individuals in a sufficiently immunized population means that transmission of infection will decline and may cease altogether. Herd immunity effectively establishes a barrier of immune individuals that prevents access of the transmissible agent to susceptible individuals (Figure 24.18). The general aim of public health in guarding against communicable disease is to establish and maintain effective herd immunity through mass immunization.

In vitro ________________ is done to establish the sensitivity of a particular microorganism to a panel of select antimicrobial drugs and is used to determine which antimicrobial drug will be most successful in treating a bacterial infection in vivo. Susceptibility patterns of various bacteria and fungi are examined by both in vitro and in vivo tests

susceptibility testing

minimal inhibitory concentration (MIC)

the lowest concentration of the drug that will prevent the growth of an organism

vaccine coverage rate

the proportion of a population that must be immunized to achieve herd immunity

Para-aminobenzoic acid (PABA)

the substrate for an enzymatic reaction leading to the synthesis of folic acid

Phages tend to be relatively minor contributors in the transfer of resistance genes between unrelated species. Like most viruses, phages tend to be quite host-specific. However,______________ can transfer resistance genes onto plasmids, as do many transposable elements. When this happens, the plasmid need only be transferred through conjugation or transformation to confer resistance in other species.

transducing phages

what is an polio virus and how can we get it

transmitted via the fecal-oral route. Humans ingest the virus from contaminated water and food or through contact with feces-contaminated objects. The virus attaches to a specific immunoglobulin-like receptor known as the poliovirus receptor, which is present on epithelial cells lining the pharynx and small intestine and also on leukocytes and motor neurons. Viral replication in epithelial cells of the digestive tract results in shedding of large numbers of infectious virions. In areas where poliovirus is still endemic (see Figure 18.30), it can be found in lakes, rivers, and streams contaminated by raw human sewage.

The Changing Patterns of Polio: You might expect that in an unvaccinated population where high levels of endemic poliovirus exist, the incidence of poliomyelitis, and therefore the incidence of paralytic poliomyelitis, would be high. However, this is not the case. Poliovirus has existed in human populations for thousands of years, yet the incidence of poliomyelitis has risen to become a public health concern only within the past century. Before modern sanitation facilities and vaccination programs, poliovirus commonly infected very young children. Often, these children did not develop poliomyelitis because their naturally immune mothers, who were also exposed to the virus and survived, provided them with ____________________ from breast milk). When infected with poliovirus during this time of passive immunity, they were protected from disease by this transferred antibody. Thus, poliovirus was usually only able to cause mild infection, allowing these children to develop their own adaptive immunity to the virus. By the age of five years, most children had acquired adaptive immunity to the virus and were protected from subsequent infections in their later years, greatly lessening the risk of poliomyelitis (Figure 24.20). Under these conditions, the incidence of poliomyelitis among infected individuals is estimated to be 0.05 percent.

transplacental IgG and secreted IgA

Reduction of Membrane Permeability Recall that one of the reasons for the development of semisynthetic β-lactams was to improve __________ across the outer membrane in Gram-negative bacteria. To counter the ability of these newer drugs to reach the periplasmic PBPs, some Gram-negative bacteria produce altered porin proteins to restrict the transport of these drugs.

transport

Although often used as a synonymous term with immunization, _____________ is the physical act of injecting or inoculating the immunizing substance into the body.

vaccination

The herd immunity threshold allows estimation of the____________________ the proportion of individuals in the population that must be vaccinated to achieve herd immunity. Usually vaccine coverage target rates are set to slightly exceed herd immunity thresholds, to account for the usually low rates of vaccine failure. For example, the WHO sets vaccine coverage target rates at 95 percent for measles, which has a maximum herd immunity threshold of 94 percent

vaccine coverage rate,

Inhibitors of Peptidoglycan Synthesis Members of the_______________ class of antibacterial drugs share the common structural feature of the β-lactam ring required for binding to a group of wall-associated proteins known as__________________. The PBPs have a transpeptidase function, required for joining together the subunits that make up peptidoglycan. Penicillins and cephalosporins are among the most commonly prescribed drugs, accounting for almost 50 percent of antibiotics used.

β-lactam penicillin-binding proteins (PBPs)

Vaccine design: DNA vaccines

• Consist of a cloned gene(s)in a DNA vector. • Delivered to cells by injection,engineered virus,or electroporation • If the gene is picked up and expressed,it stimulatesa protective immune response. -Similar to recombinant subunit vaccines -Benefit is longer exposure (stronger response). -Vaccines don't contain live microbes, avoiding the dangers associated with attenuated vaccines • Largely still experimental. • None available yet for use in human beings

Vaccine design: Inactivated vaccines

• Consist of whole virus/cells that have been inactivated by heat or chemicals. Benefits include that the microbe can't revert, can't replicate, and can't spread. Drawbacks include lower/shorter stimulation of immune responses, a need for multiple injections, and greater risk of negative side effects.

Vaccines

• Edward Jenner coined the term vaccination in 1798 with his work protecting people from smallpox. Jenner observed that milkmaids had smallpox less frequently than the rest of the population but they would get a lesser disease (cowpox). He purposely infected a boy with cowpox, then later with smallpox—and the boy didn't develop the disease.

Naturally occurring penicillins bind to penicillin-binding proteins (PBPs) in microbes for transport into the cell.

• Gram-negative cell PBPs are in the periplasm, where the drugs typically can't reach. • Chemical modification of the drugs can make them more likely to reach the periplasm, increasing the spectrum to Gram-negative cells. • Gram-positive cell PBPs are in the outer cell wall, making them more susceptible to such drugs.

Principles of epidemic control The triangle of epidemiology allows us to conceptualize the three major factors associated with development of an epidemic:

• Host factors • Agent factors • Environmental factors

Antiviral drugs Tricky because viruses often use host cell processes

• If you inhibit a host cell process,toxicity will be high

Polio vaccines: Inactivated vs. Attenuated

• Inactivated polio vaccine (IPV, "Salk" vaccine) began being used in 1955. • Oral polio vaccine (OPV, "Sabin" vaccine) began being used in the 1960s.

Antiviral drugs Common mechanisms involve

• Inhibition of nucleic acid synthesis Often through nucleoside/nucleotide analogues (AZT, acyclovir) Inhibition of virus life-cycle steps

Intrinsic drug resistance due to natural structure or metabolism

• May be due to lack of access to a target/transport molecule (as in Gram-negative PBPs)• May be due to a side-effect of normal function (efflux pumps in fungal cells)

Polio vaccine issues

• OPV-is"better"atstimulating immunity, but: • It can revert and cause illness. • It requires more careful handling and refrigeration. IPV -is"worse"at stimulating immunity,but • It can't revert. For global eradication of polio,all immunization will eventually have to shift back to IPV instead of OPV.

Natural selection and drug resistance • Microbes will genetically adapt to drug selective pressures through

• Random mutations • Recombination • Horizontal gene transfer mechanisms Most changes will have no effect on drug resistance. However, with continued drug therapy (continued selective pressure), rare genetic changes can quickly produce a dominant resistant microbe strain.

Combating drug resistance How do we prevent the development of antimicrobial resistance? Stopping the use of antimicrobials is out of the question, but there are several approaches that, if used collectively, may help to reduce the emergence of resistance.

• Reduce use •Use selective drugs •Use multidrug cocktails •Use effective infection control •Develop new vaccines and improve access •Develop alternatives/develop drugs in a smarter way

Modification of preexisting genes

• Spontaneous mutations(polymeraseerrors) • Exposure to mutagens inducing mutations

Antibacterial drugs: Inhibitors of nucleic acid synthesis

• Sulfonamides are a structural analog (and competitive inhibitor) for PABA- used to make folic acid, a necessary precursor for synthesis of nitrogenous bases.

acquiring new genes conferring resistance through

• Transposable elements • Transducing phages • Plasmid transfer (transformation/conjugation) • Recombination events

Vaccine Design

• Vaccines confer protection by initiating immune memory. • Specialized T and B cells that are produced post- stimulation The ideal vaccine generates a high level of immune memory without serious side effects. Different types of vaccines include • Attenuated • Inactivated • Subunit • DNA

Resistance genes in the environment

• When one microbe in a complex microbial ecosystem secretes a natural antibiotic compound, it can provide a selective pressure on other microbes.• This is microbial warfare in a natural setting (without human intervention).

Susceptibility testing: Dilution susceptibility test

•Dilutions of a particular antimicrobial drug are incubated with a standardized culture of the test organism for a given period of time •The lowest concentration of the drug resulting in no grow this the MIC