Microbiology Ch. 12

Drugs that inhibit the nucleic acid synthesis

-Acyclovir (Zovirax). -Valacyclovir (Valtrex). -Famicicilovir (Famvir). -Penicicilovir (Denavir). -AZT (Azidothymidine).

Principles of Antimicrobial Therapy

-Administer a drug to an infected person that destroys the infective agent without harming the host's cells. (human) -Antimicrobial drugs are produced naturally or synthetically.

Inhibition of viral entry

-Amantadine (rimantadine): Used in the treatment of influenza A infections. -Tamiflu (Relenza): used in the treatment of influenza infections.

Antibiotics that inhibit protein synthesis

-Aminoglycosides. -Tetracyclines. -Chloramphenical.

Macrolide Polyenes:

-Amphotericin B. -Nystatin.

Interactions between Drugs and the Microbe

-Antimicrobial drugs should be selectively toxic: drugs should kill or inhibit microbial cells without simultaneously damaging the host tissues (eukaryotic cells leads to toxicity). -As the characteristics of the infectious agent become more similar to the vertebrate host cell, complete selective toxicity becomes more difficult to achieve and more side effects are seen.

Antibacterial Drugs: antibiotics that inhibit cell wall synthesis, but interfere with peptidoglycan formation:

-Beta-lactam antimicrobials: all contain a highly reactive beta-lactam ring. -Primary mode of action is to interfere with cell wall synthesis. -Greater than 1/2 of all anti-microbic drugs are beta-lactams. -Penicillins and cephalosporins most prominent beta-lactams.

Additional beta-lactam drugs

-Carbapenems: imipenem: broad spectrum drug for infections with aerobic and anaerobic pathogens; low dose, administered orally with few side effects. -Monobactams: Aztreonam: Narrow spectrum drug for infections by gram-negative aerobic bacilli; may be used by people allergic to penicillin.

Major side effects

-Estimate that 5% of all persons taking anti-microbials that will experience a serious adverse reaction to the drug: side effects. Major side effects:. Direct damage to tissues or toxicity: liver or kidney. Allergic reactions: hives or anaphylaxis rash. Disruption in the balance of normal flora: development of a superinfection. Examples: diarrhea and a vaginal yeast infections.

Antibiotics that act on DNA or RNA

-Fluroroquinolones. -Rifampin.

Identifying the agent

-Identification of infectious agent should be attempted as soon as possible. -Specimens should be taken before anti-microbials are initiated.

The MIC and Therapeutic Index

-In vitro activity of a drug is not always correlated with in vivo effect. If therapy fails, a different drug, combination of drugs or different administration must be considered. Best to choose a drug with highest level of selectivity, but lowest level toxicity: measured compared to its minimum effective dose. High index is desirable.

Testing for drug susceptibility

-Kirby Bauer technique. -E-test diffusion test. -Tube dilution technique.

Penicillin family

-Large diverse group of compounds with 'cillin'. Could be synthesized in the laboratory. Produced by Penicillium chrysogenum. Natural and semisynthetic. All consist of 3 parts: Thiazolidine ring, Beta-lactam ring, Variable side chain dictating microbial activity. Penicillins G and V are the most important natural forms. Penicillins that do not penetrate the outer membrane of gram negative bacterial are less effective.

Natural Selection and Drug resistance

-Large populations of microbes likely to include drug resistant cells due to prior mutations or transfer of plasmids: no growth advantage until exposed to drug. If exposed, sensitive cells are inhibited or destroyed while resistance cells will survive and proliferate. -Eventually population will be resistant: natural selection.

Outbreak at Bluebell Creamery

-Listeria monocytogenes.

Creating Drug resistant microbes (drug abuse)

-Over prescribed. -Drugs prescribed without culture or susceptibility test. -Broad spectrum usually prescribed. -Regulations in other countries. -Patients do not finish drugs or take later. -Antibiotics used in livestock.

Antibiotics that inhibit cell wall synthesis, but interfere with peptidoglycan formation

-Penicillin family. -Cephalosporin family.

Characteristics of the idea Antibiotic

-Selectively toxic to the microbe, but non-toxic to the host cells. -Microbicidal rather than microbistatic. -Soluble and functions when diluted in body fluids. -Remains potent long enough to act/ not broken down or excreted prematurely. -Not subject to development of drug resistance. -Complements host's defenses. -Readily delivered to site of infection. -Not excessive in cost. -Does not cause allergies or predispose host to other infections.

Mechanisms of Drug resistance

1. Drug inactivation: produce enzyme that destroys the drug. 2. Decreased permeability: it changes receptor for the drug. 3. Activation of drug pumps: that pumps the drug out of the cell. 4. Change at the drug binding site: on the ribosome. 5. Use of alternate metabolic pathway: make new enzymes to create alternate chemical pathway to end product.

The Acquisition of drug resistance

Adaptive response in which microorganisms begin to tolerate an amount of drug that would ordinarily be inhibitory; due to genetic versatility or variation; intrinstic and acquired. Acquired resistance: Mutations in critical chromosomal genes: Spontaneous mutations, induced mutations, transposons. Acquisition of new genes or sets of genes via transfer from another species: conjugation, transduction, or transformation.

Flucytosine

Analong of cytosine. -Used for cutaneous mycoses or in combination with amphotericin B for systemic mycoses.

Origins of Antimicrobial Drugs and History

Antibiotics are common metabolic products of aerobic bacteria and fungi. -Bacteria in genera: Streptomyces and Bacillus. -Molds in genera: Penicillium and Cephalosporium. By inhibiting the other microbes in the same habitat, antibiotic producers have less competition for nutrients and space.

Antimicrobial drugs mechanism: Nucleic acid

Antimicrobial drugs can block the synthesis of nucleotides and inhibit replication. Examples: chloroquine and antiviral drugs like AZT.

Chemotherapeutic agent

Any chemical used in the treatment, relief, or prophylaxis of a disease.

Inhibition of viral assembly or release: Saquinavir: protease inhibitor

Block enzyme needed to assemble virus particles. Used to treat AIDS.

Antibiotics that act on DNA or RNA: Fluroroquinolones

Broad-spectrum effectivenes. Synthetic drug made in the lab. Now a family of fluroquinolones: ciprofloxacin ("cipro") for anthrax and levafloxacin (levaquin). Used for UTI's, STD's, gastrointestinal infections, respiratory infections, soft tissue infections. Primary problems: headache, dizziness, tremors, and GI distress. Concerns have arisen regarding the overuse of quinoline drugs. CDC is recommending careful monitoring of their use to prevent ciprofloxacin-resistant bacteria.

Tetracyclines

Broad-spectrum produced by streptomyces. Examples: doxycycline and minocycline. Treatment for STD's, Rocky mountain spotted fever, lyme disease, typhus, acne, and protozoa. Generic tetracycline is low in cost, but limited by its side effects. Primary problems: GI disruption, staining of teeth, and fetal bone development.

Synthetic azoles

Broad-spectrum. Over the counter creams, sprays, or lotions; or prescribed for ring worm, jock itch, athelete's foot, and vaginal yeast infection. For dermatophyte or mucous membrane infection. Include: ketoconazole, clotrimazole and miconazole. Found in over the counter products: Gyne-Lotrimin, Monistat, Micatin, and Nizoral.

Second generation of Cephalosporin

Cefaclor, cefonacid: most effective against gram-negative bacteria.

Fourth generation of Cephalosporin

Cefepime: widest range; both gram-negative and gram-positive.

Third generation of Cephalosporin

Cephalexin, cefriaxone: broad-spectrum activity against enteric bacteria with beta-lactamases.

First generation of Cephalosporin

Cephalosporin, cefazolin: most effective against gram-positive cocci and few gram-negative.

Antimicrobial drugs mechanism: Cell membrane

Damage to the cell membrane causes disruption in metabolism or lysis. These drugs target a special type of lipids. Examples: polymyxins, amphotericin B, and nystain.

Anti-viral drugs

Developed to target specific points in the infectious cycle of viruses. Three major modes of action: Inhibition of viral entry (it stops the growth of cell wall), Inhibition of nucleic acid synthesis (blocking replication, transcription, and/or translation) (it stops the growth of RNA/DNA). -Nucleotide analogs: adenine, guanine, cytosine, and thymine. Inhibition of viral assembly or release.

Synthetic antibiotic

Drug is made totally in the lab.

Semisynthetic antibiotic

Drug that is chemically modified in the lab after being isolated from the natural source.

Chloramphenical

Entirely synthesized through chemical processes. Very toxic, restricted uses can cause irrevesible damage to bone marrow. Typhoid fever, brain abscesses, rickettsial, and chlamydial infections. Primary problems: injury to red and white blood cell pre-cursors. It causes fatal aplastic anemia.

Antifungal drugs

Fungal cells are eukaryotic; a drug that is toxic to fungal cells also toxic to human cells. Divided to treat two types of disease: superficial and systemic. Mode of action: Most antifungals act by disrupting the cell membrane of the fungus. Systemic affects the lungs and superficial affects the skin and mucous membranes.

Cephalosporin family

Generic names have root -cef, ceph, or kef. Produced byCephalosporium acremonium. Natural and semisynthetic. All consist of 3 parts: Six carbon ring, Beta-lactam ring, Two variable side chains dictating microbial activity. Relatively broad-spectrum, resistant to most penicillinases, and cause fewer allergic reactions. Some are given orally; many must be administered parenterally. Five generations exist: each group more effective against gram-negatives than the one before with improved dosing schedule and fewer side effects.

Acyclovir, Valacyclovir, Famicicilovir, and Peniciclovir

Guanine analog. Used to treat herpes infections. Also, used to treat chickenpox and the shingles.

Considerations in Selecting an Anti-microbial drug

Identify the microorganism causing the infection. -should be attempted as soon as possible. -Specimens should be taken before anti-microbials are initated. Test the microorganism's susceptibility (sensitivity) to various drugs in vitro when indicated. The overall medical condition of the patient.

E-test diffusion test

Lawn of bacteria made on petri plate. E-test strip placed on top with decreasing concentration of antibiotic; used in research.

Inhibition of nucleic acid synthesis

Many anti-viral agents mimic the structure of nuclecotides and compete for sites on replicating or synthesizing DNA.

Anti-helminthic drugs

Mebendazole. Used to treat worm infections: roundworm and tapeworm.

Anti-protozoan drugs

Metronizdazole or Flagyl. Used to treat several parasitic infections such as mild to severe intestinal infections and an STD. Side effects: Black-hairy tongue: it breaks down from the hemoglobin placed in the papillae of the tongue.

Kirby-Bauer technique

Most commonly done in hospitals and labs that identify what antibiotics that will work for the causative agent. Disk diffusion test. Procedure: make a lawn of bacteria on a petri plate, place different antibiotic disks on lawn and observe growth after 24-48 hours, look for zone of inhibition around antibiotic discs. Measure zone and compare to chart for susceptibility or resistance.

Antibiotics that block Metabolic pathway: Sulfonamides

Most important are sulfonamides or sulfa drugs: first antimicrobic drugs. Most are synthetic. Narrow-spectrum; block the synthesis of folic acid by bacteria. Sulfisoxazole: shigellosis, UTI, protozoan infections. Silver sufadiazine: burns and eye infections. Trimethoprim: given in combination with sulfamethoxazole: UTI, PCP. Primary problems: rash, formation of crystals in kidney, hemolysis of RBC's, reduce platelets.

Antimicrobial drugs mechanism: Protein synthesis

Most inhibitors of translation react with the ribosome-mRNA complex. Two possible targets are the 30S sub-unit and the 50S sub-unit. Examples: streptomycin, gentamicin, tetracyclines, chloramphenicol, and erythromycin.

Amphotericin B

Most versatile and effective; works on most fungal infections. Topical: skin and mucous membrane infections: yeast infections. Systemic fungal infections: "Gold Standard".

Antibiotics that damage bacterial cell membranes: Polymixins

Narrow spectrum peptide antibiotics with a unique fatty acid component. Treats drug resistant Pseudomonas aeruginosa and severe UTI's; used in topical ointment.

Non Beta-lactam Cell wall inhibitors: Vancomycin

Narrow spectrum used to treat resistant Staphylococcus and Enterococcus. Toxic and hard to administer; restricted use. MRSA: methicillin resistant. VRE: vancomycin resistant. VRSA: vancomycin resistant. Primary problems: nausea, rash, dizziness, kidney damage, hearing loss (ringing in the ears).

Rifampin

Narrow spectrum; produced by Streptomyces. Mostly used to treat tuberculosis and leprosy. Prophylaxis of Neisseria meningitdis carriers. Also used to treat Legionella, Brucella, and Staphylococcus infections.

Echinocandins

New category of antifungal drugs. Damage cell walls of fungal cells so not toxic to human cells. First licensed: capsofungin. Used for invasive fungal disease (aspergillosis) where the patient cannot take amphotericin B or flucytosine. Used on invasive yeast infections.

Antimicrobial drugs mechanism: Cell wall

Peptidoglycan in the cell wall helps provide a rigid structure that protects against changes in the osmotic pressure in the environment. Active cells are constantly producing more peptidoglycan to place in the cell wall. Some drugs interfere with one or more of the enzymes required to compete with this process, causing the cell to develop weak points at growth sites and to become osmotically fragile. Example: penicillin and cephalopsporins.

Macrolide and Related Antibiotics: Erythromycin

Produced by Streptomyces. Broad-spectrum, fairly low toxicity. Taken orally for Mycoplasma pneumonia, legionellosis, Chlamydia, pertussis, diphtheria and as a prophylactic prior to intestinal surgery. For penicillin-resistant: gonococci, syphilis, acne. Newer semi-synthetic macrolides: clarithromycin and azithromycin (zithromax). Primary problems: stomach pain, nausea, diarrhea, and rash.

Antibiotics that inhibit protein synthesis: Aminoglycosides

Produced by: Streptomyces: streptomycin and Micromonospora: gentamicin. Broad spectrum, inhibit protein synthesis, especially useful against aerobic gram-negative rods. Streptomycin: bubonic plague and tuberculosis, Gentamicin: less toxic used against gram-negative rods. Newer: tobramycin and amikacin. Neomycin: Primary problems: diarrhea, hearing loss, dizziness, kidney damage.

Natural antibiotic

Product is unchanged from organism that produces it; Streptomyces is the most prolific producer.

Antimicrobial drugs mechanism: Cytoplasm

Some drugs act as an analog to an enzyme in a metabolic pathway. Examples: sulfonamides and trimethoprim.

Griseofulvin

Stubborn cases of dermatophyte (ringworm) infections that are harder to treat. -Drug gets deposited in the skin, hair and nails. -Nephrotoxic: kidney and liver.

MIC: Minimum inhibitory concentration

The smallest concentration of drug that visibly inhibits growth.

AZT: Azidothymidine

Thymine analog. Used to treat AIDS.

Anti-parasitic drugs: Antimalarial drugs: Quinine

Used for hundreds of years for treatment of malaria. Extracted from the bark of the cinchona tree: "fever tree". Replaced by synthetics called quinolines. Include: Chloroquine, Primaquine, and mefloquine (used for chloroquine-resistant stains).

Nystatin

Used topical or orally for treatment of yeast infections and diaper rash.

Broad spectrum

a drug that will kill a wide range of bacteria. Example: Tetracycline.

Narrow spectrum

a drug that will only kill a few types of bacteria. Example: penicillin.

Alexander Fleming

accidentally discovered penicillin.

Semisynthetic penicillins

ampicillin, carbenicillin, and amoxicillin have broader spectra: Gram-negative infections. Broad spectrum penicillins can cross the cell membranes of gram negative bacteria. Penicillinase-resistant: methicillin, nafcillin, cloxacillin. Can add beta-lactamase inhibitor such as clavulanic acid. Clavulanic acid and amoxicillin = augentin (clavamox). Penicillin is the drug of choice for gram-positive cocci (streptococci) and some gram-negative bacteria (meningococci and syphilis spirochete). Primary problems: allergies (rash) and resistant strains of bacteria.

Paul Ehrlich

coined the term chemotherapy to describe chemicals that would act as "magic bullets" and selectively kill pathogens.

RMS: Red man syndrome

flushing of the face, neck and upper torso.

In vitro

in the lab.

In vivo

in the person.

Gerhard Domagk

introduced the first sulfa drug.

Antibiotic

is a metabolic product of one microorganism that inhibits or destroys other microorganisms.

Tube dilation technique

more sensitive and quantative. Serial dilutions of antibiotics in test tubes. All tubes inoculated with the same amount of bacteria. This test can determine the minimum inhibitory concentration (MIC).

Bacitracin

narrow spectrum produced by a strain of Bacillus subtilis; used topically in ointment. Triple antibiotic ointment.

Cephalosporin primary problems

rash, nausea, and diarrhea.

Howard Florey and Ernst Chain

worked out how to industrially produce penicillin.

Isoniazid (INH)

works by interfering with mycolic acid synthesis; used to treat infections with Mycobacterium tuberculosis.