Ch 20 Antibiotics
Polymyxin B
Bactericidal against gram neg bacteria by binding to cell membranes. This binds to eukaryotic cells as well to a lesser extend which limits use to topical applications. Commmon ingredient in first aid skin ointments.
Ethambutol
Inhibits enzymes required for synthesis of other mycobacterial cell wall components of mycobacterial cell wall.
Iodoquinol
Mechanism unknown; porly absorbed but taken orally to eliminate amebic cysts in intestine.
Neuraminidase Inhibitors Zanamivir (inhail), Oseltamivir (oral)
Used to treat influenza virus infections. Inhibit neuraminidase, enzyme encoded by influenza viruses. Enzyme important for release of viral proteins from infected cells. Both shorten infection when taken within 2 days of onset system
Penicillins
A family of antibacterial medications; different groups vary in their spectrum of activity and their susceptibility to beta-lactamases
Streptogramins Quinupristin + Dalfopristin = Synercid
A synergistic combination of 2 drugs that bind to 2 different sites on the 50S ribosomal subunit, inhibiting distinct steps of protein synthesis aka translation and both drugs together called Synercid. Individually each drug is bacteriostatic, but together they are bactericidal. Effective against a variety of gram pos bacteria including some resistant to beta lactam and vanco.
Phenols Niclosamide
Absorbed by cestodes in intestinal tract, but not human host
Nitroimidazoles Metronidazole
Activated by metabolism of anerobic organisms. interfere with electron transfer and alter DNA. Do not reliably eliminate cyst stage. Metronidazole also used to treat infections by anaerobic bacteria
Narrow spectrum antimicrobials
Affect limited range of bacteria and use required pathogen to be identified and antimicrobial susceptibility tested. Cause less disruption to normal microbiota.
Broad spectrum antimicrobials
Affect wide range of bacteria. Drugs are important for acute life threatening diseases when immediate antimicrobial therapy is need and no time to culture to identify pathogen. Disadvantage is they disrupt normal microbiota that play role in excluding pathogens which can lead pt predisposed to other infections.
Examples of emerging antimicrobial resistance - Mycobacterium Tuberculosis
Always long and complicated, requiring combo of 2 or more diff drugs taken for 6 months or more. Mycobacterium tuberculosis easily becomes resistant to first line drugs through spontaneous mutation. Large numbers of bacterial cells are found in active infection so likely 1 cell has developed spontaneous resistance to drug, which why combo therapy is used. Length of treatment is due to slow growth of TB. Many TB pts non complient with combo therapy, skipping dose or stop treatment so strains have developed resistance to first line drugs. Results in longer more expensive treatment that are less effective. Strains resistant to 2 favored drugs - Isoniazid and Rifampin- called Multidrug resistant M. tuberculosis MDR-TB. To prevent emergency cities use direct observed therapy where healthcare workers visit pts and watch take drugs make sure they comply with treatment. Strains of extensively drug resistant M. tuberculosis XDR-TB even greater concern. Defined as TB strains resistant to isoniazid, rifampin, plus 3 ore more second line drugs.
Antibaterial medications that interfere with Protein Synthesis
Aminoglycosides, Tetracyclines, Glycyclines, Macrolides, Chloramphenicol, Lincosamides, Oxazolidinones, Streptogramins Inhibit prokaryotic protein synthesis by binding to prokaryotic 70s ribosome, either 30S or 50s subunit, making different than eukaryotic 80s ribosome so suitable target for selective toxicity. Mitochondria of eukaryotic cells also have 70s ribosome so partially account for toxcicity
Examples of emerging antimicrobial resistance - Staphylococcus aureus
Another common cause for healthcare associated infections, most resistant due to acquisition of gene encoding penicillinase. Until recently , strains treated with methicillin or other penicillinase resistant penicillins. New strains emerged that not only produce penicillinase, but also PBP with low affinity for all beta lactam drugs. These drugs called methicillin resistant staphylococcus aureus MRSA,resistant to methicillin and beta lactam drugs. 2 Categories for MRSA, healthcare associated HA-MRSA and comunnity acquired CA-MRSA. HA generally resistant to wide range of antimicrobial meds so treated with Vancomycin. Some reports that are vancomycin intermediate S. aureus VISA and vancomycin resistant S. aureus VRSA. CA-MRSA currently susceptible to antibiotics other than beta lactam
Polyenes Amphotericin B, Nystatin
Antibiotics produced by certain species of Streptomyces. Bind to ergosterol, disrupting plasma membrane and allowing cytoplasm to leak out. Amphotericin B is very toxic but most effective for treating life threatening infections; newer lipid based emulsions are less toxic but expensive. Nystatin is too toxic for systemic use, use topically.
Mechanisms of acquired resistance - alteration in target molecule
Antimicrobial drugs generally act by recognizing and binding to specific target molecules in bacterium, interfering with function. Minor structural changes in target can prevent drug binding. Mods in penicillin binding proteins PBP prevent beta lactam drugs from binding. Change in ribosomal RNA, target for macrolids, prevents drugs from interfering with ribosomal function.
Resistance to Antimicrobial drugs
Antimicrobial drugs inced use and misused, resistant bacterial strains selective advantage over sensitive counterparts. Alarming because impact on cost, complications, outcomes of treatment. Dealing requires understanding of mechanisms of resistance and how are spread.
Antibiotics
Antimicrobial drugs naturally produced by microorganisms. ex; of antibiotics coming from microorganims that normally reside in soil Streptomyces, Bacillus (bacteria), Penicillium and Cephalosporium (fungi)
Tissue distribution, metabolism, excretion of drug
Antimicrobs differ not only in their action and activity, but also in how distributed, metabolized, and excreted by the body. Only some drugs cross from blood into cerebrospinal fluid. Drugs that are unstable at low pH are destroyed by stomach acid, so drugs administered intravenous or intramuscular. Half life of drug is time it takes for serum concentration to decrease by 50% and dictates frequency of doses required to maintain effective level in body. Pts with kidney or liver dysfunction often excrete or metabolize drugs more slowly so drug doseages must be adjusted to avoid toxic levels.
Acquisition of resistance - Spontaneous Mutation
As cell replicates, mutations happen at low rate, but can have significant effect on resistance of bacterial population to antimicrobial drug. Acquired resistance to streptomycin (aminoglycosaide) is example of consequence of spontaneous mutation. Single base pair change in gene coding ribosomal protein alters target enought to make cell streptomycin resistant. When antimicrobial drug has several different targets or multi binding sites on single target, resistance via spontaneous mutation less likely because several diff mutations required to prevent binding of drug.. Newer aminoglycosides bind several sites in ribosome, so resistance from mutation unlikely. Drugs like streptomycin for which single point mutation causes resistance are sometimes used in combo with other drug. If cell spontaneously develops resistance to one drug, another will kill. Combination therapy is effective because chance of cell simultaneously develop mutation resistance to multi drugs is low.
Intestinal and Tissue Helminth drugs
Avermectins, Benzimidazoles, Phenols, Piperazines, Pyrazinoisoquinolines, Tetrahydropyrimidines
Intrinsic (innate) resistance
Bacteria are inherently resistant to effects of some drugs. Ex; Mycoplasma species lack cell wall, so resistant to penicillin and any drug interfering with peptidoglycan synthesis. Many gram neg bacteria intrinsically resistant to certain drugs because lipid bilayer of outer membrane prevents drug from entering.
Beta-Lactam drugs
Bactericidal against a variety of bacteria; competitively inhibit penicillin-binding proteins pr enzymes that catalyze formation of peptide bridges between adjacent glycan strands, essential step of peptidoglycan synthesis.This causes disruption in cell wall synthesis which weakens wall, causing cell to lyse. These are bactericidal only against growing bacteria, because these cells continuously synthezies peptidoglycan. Resistance is due to synthesis of beta-lactamases, decreased affinity of penicilliin-binding proteins, or decreased uptake. All share chemical structure called beta-lactam ring. Different beta lactam vary in spectrum of activity because of cell wall structure. Peptidoglycan of gram post is exposed to outside environment, so drugs can directly contact enzyme that synthezie molecule. Gram neg prevent drugs from accessing target. PBPs vary of gram pos to gram neg. PBPs of obligate anerobes differ from aerobes. Various PBPs have different affinties for beta drugs that can exist among related organisms!
Fluoroquinolones Ciprofloxacin, Moxifloxacin
Bactericidal against a wide variety of gram pos and gram neg bacteria; Synthetic drug that inhibit 1 or more enzymes called topoisomerases, which maintain supercoiling of DNA in bacterial cell. One topoisomerase, DNA gyrase, breaks and rejoins strands to relieve strain caused by localized unwinding of DNA during replication and transcription so inhibition prevents essential cell processes. Resistance is most often due to structural alterations in topoisomerase target like in DNA gyrase target.
Rifamycins Rifampin
Bactericidal against gram pos and some gram neg bacteria. Bind to prokaryotic RNA polymerase, blocking the initiation of RNA synthesis. Primarily used to treat infections caused by Mycobacterium tuberculosis and as prophylaxis for patients who have been exposed to Nesseria meningitidis. Resistance develops rapidly and is due to mutation in gene that encodes RNA polymerase.
Daptomycin
Bactericidal against gram pos bacteria by inserting into bacterial cytoplasmic membranes causing bacterial membrane damage. Not effective against gram neg because it cannot penetrate outer membrane.
Vancomycin
Bactericidal against gram pos bacteria; blocks peptidoglycan synthesis by by binding to peptide side chain of NAM molecules being assembled to form glycan chains which weakens cell causing lysis. Drug poorly absorbed from intestinal tract so given IV unless used for intestinal infections. Important to treat gram pos bacteria resistant to beta lactam drugs.Used to treat serious systemic infections and severe Clostridium difficil an associated disease that do not respond to Metronidazole. Does not cross outer membrane of gram neg bacteria so organisms are intrinsically resistant. Acquitred resistance to vanco is problem, and mostly due to change in peptide side chain of NAM molecule preventing Vanco from binding. . In enterococci, resistance is due to a plasmid-encoded altered target.
Bacitracin
Bactericidal against gram pos bacteria; interferes with cell walll biosynthesis by interfering transport of peptidoglycan precursors across cytoplasmic membrane. Toxicity limits use to topical applications. Common ingredients in non prescription antibiotic ointments.
Aminoglycosides; Streptomycin, Gentamicin, Tobramycin, Amikacin, Neomycin
Bactericidal drug against aerobic and facultative bacteria; bind to 30S ribosomal subunit, blocking the initiation of translation and causing the misreading of mRNA by ribosomes. Generally not effective against anaerobes, enterococci, and streptococci because they enter bacterial cells by active transport process that requires respiratory metabolism. To extend spectrum, aminoglycosides use in synergistic combo with beta lactam drug which interferes with cell wall synthesis allowing aminoglycoside to enter cells that would otherwise be resistant. Resistance is due to a plasmid encoded inactivating enzyme, alteration of the target molecule, or decreased uptake by a cell. Neomycin is commonly used in non prescription topical antibiotic ointments. Form of Tobramycin administered through inhalation makes treatment of lung infections in cystic fibrosis pts cause by Pseudomonas aeruginosa safer and more effective. All very toxic causing hearing loss and kidney damage; used when other alternatives not available.
Sulfonamides aka Sulfa drugs
Bacteriostatic against a variety of gram pos and gram neg bacteria. Structurally similar to para-aminobenzoic acid PABA, substrate in pathway for folate biosynthesis. Because similar, enzyme that normally binds PABA binds sulfa drugs instead (competitive inhibition). Human cells lack enzyme, providing basis for selective toxicity of sulfonamides. Resistance is most commonly due to a plasmid encoded enzyme that has lower affinity for drug.
Oxazolidinones Linezolid
Bacteriostatic against a variety of gram pos bacteria. Bind to the 50S ribosomal subunit, interfering with the initiation of protein synthesis aka translation. Useful in treating infections caused by bacteria that are resistant to beta lactam drugs, and vancomycin.
Lincosamides Lincomycin, Clindamycin
Bacteriostatic against a variety of gram post and gram neg bacteria, including the anaerobe Bacteroides fragilis. Bind to the 50S ribosomal subunit, preventing the continuation of protein synthesis aka stop translation. Useful for treating infection from intestinal perforation because they inhibit Bacteroides fragilis, member of normal intestinal microbiota that frequently resistant to ther antimicrobials. Associated with an even greater risk of developing Clostridium difficil- associated disease because most C. diff strains are resistant to lincosamides.
Macrolides Erythromycin, Clarithromycine,Azithromycin
Bacteriostatic against many gram pos bacteria as well as the most common causes of atypical pneumonia; bind to 50S ribosomal subunit, preventing the continuation of translation. Used for treating patients who are allergic to beta lactam drugs. Drug of choice if allergic to Penicillin. Not effective against Enterobacteriaceae because do not pass outer membrane. Resistance is due to modification of ribosomal RNA target, production of enzyme that chemically modifies drug, alteration of the target molecule, or alterations that result in decreased uptake by a cell.
Chloramphenicol
Bacteriostatic and broad spectrum ; binds to the 50S ribosomal subunit, preventing peptide bonds from being formed aka blocking translation.Inhibits variety of gram neg and pos but generally used only as last resort for life threatening infections in order to avoid aplastic anemia or inability of body to form WBC and RBC. Resistance is often due to a plasmid encoded inactivating enzyme.
Selective toxicity
Causing greater harm to microbs than to human host done by interfering with essential structures or biochemical processes that are common in microbs but not humans.
Targets of Antibacterial Medications
Cell wall (peptidoglycan synthesis) - beta lactam drugs, vancomycin, bacitracin Cell membrane integrity - Polymyxin B, Daptomycin Metabolic pathways (folate biosynthesis) - Sulfonamides, Trimethoprim Nucleic acid synthesis - Fluoroquinolones, Rifamycins Protein synthesis - Aminoglycosides, Tetracyclines, Macrolides, Chloramphenicol, Lincosamides, Oxazolidinones, Streptogramins
Chemotherapeutic agents
Chemicals used to treat disease
Antimicrobial drugs; antimicrobials
Chemicals used to treat microbial infections
Combinations of antimicrobial medications
Combos used to treat infections, but must choose carefully because some counteract others. ex; bacteriostatic drugs interfere with drugs that kill only actively dividing cells. Antagonistic - Counteracting combinations Synergistic - Combos where activity of one drug enhances activity of the other. Addititve - Combos that are neither synergistic nor antagonistic
Antibacterial medications that interfere with Cell Membrane Integrity
Daptomycin, Polymyxin B Damage bacterial membranes causing cell to leak, leading to cell death.
Cephalosprins; (1st gen Cephalexin, Cephradine,) 2 Cefaclor, 2 Cefprozil,3 Cefixime,3 Ceftibuten, 4 Cefepime
Derived from antibiotic produced by fungus Acremonium cephalosporium and incuding closely related group of antibiotics made by filamentous bactera related to Streptomyces. Chemical structre makes them resistant to inactivation by certian beta lactamases. Some not effective against gram pos because has low affinity for PBPs. The later generations are generally more effective against gram neg bacteria and less susceptible to destruction by beta lactamases but susceptible to extended spectrum beta lactamases.
Bacteriostatic
Drugs inhibit bacterial growth. Pt taking bacteriostatic drug rely on bodys defence to kill or eliminate pathogen after growth has stopped. ex; Sulfa drugs are prescribed for UTI and they prevent bacteria in bladder from growing so urination can more effectively eliminate them.
Bactericidal
Drugs kill bacteria. Useful when host defenses cannot eliminate pathogens. These drugs are somtimes only inhibitory depending on drug concentrations and stage of bacterial growth. Lethal effects of different bactericidal drugs seem to involve same mechanism, regardless of drugs target. It appears antimicrobial induced cell damage overwhelms bacterias ability to detoxify reactive oxygen species, leading to extensive oxidative damage.
Slowing emergency and spread of antimicrobial resistance - Importance of educated public
Educate people about role and limitations of antibiotics. Understand antibiotics not effective against viruses. Too many people mistake antibiotics effective against viruses, and seek script for cure viral infections. Misuse selects for antibiotic resistant bacteria in microbiota. Even if not disease causing, can serve as resevoir for R plasmids transfering resistance genes to infecting pathogen.
Trypanosomes and Leishmania drugs
Eflornithine, Heavy metals, Nitrofurtimox
Replication of reverse transcribing viruses
Encode reverse transcriptase; makes DNA from RNA Retroviruses have ss+RNA genome ex; HIV Reverse transcriptase synthesizes singe DNA strand Complementary strand synthesized; dsDNA integrated into host cell chromosome Can direct productive infection or remain latent Cannot be eliminated
Antibacterial medications effective against Mycobacterium Tuberculosis Drugs
Ethambutol, Isoniazid, Pyrazinamide (other drugs include Rifampin and Streptomycin) Few drugs effective due to organisms waxy cell wall (prevents entry of many drugs) and slow growth. A group of 5 meds, first line drugs, are preferred because most effective and least toxic. Generally given in combo of 2 or more to pts who have active TB. Combo therapy decreases chance that resistant mutants will develop; if some cells develop resistance to one drug, other will eliminate. Second line drugs are used for strains resistant to first line drugs but less effective or greater risk of toxicity. First line meds try to target unique cell wall that characterizes mycobacteria.
Mechanisms of action of antifungal drugs
Eukaryotic pathogens like fungi more closely resemble human cells than bacterial which why few drugs.
Synthesis
Expression of viral genes to produce viral structural and catalytic genes ex; capsid proteins, enzymes required for replication Synthesis of multiple copies of genome Most DNA viruses multiply in nucleus Enter through nuclear pores following penetration 3 general replication strategies depending on type of genome of virus, DNA, RNA, revers transcribing .
Antibacterial medications that interfere with Nucleic Acid Synthesis
Fluoroquinolones, Rifamycins, Metronidazole. Enzymes required for nucleic acid synthesis are targets of some groups of antimicrobial drugs.
Plasmodium (Malaria) and Toxoplasma drugs
Folate antagonists, Malarone, Quinolones
Slowing emergency and spread of antimicrobial resistance - Responsibilites of pt
Follow instructions with prescriptions, even if inconvenient. Maintain adequate blood levels of antimicrobial for specific time period. If pt skips dose, blood level drug not remain high to inhibit growth of least sensitive members and if grow give rise to pop that is not as sensitive as original. Failure to complete prescribed course may not kill least sensitve organisms allowing multiplication. Misuse antimicrobials leads to resistant mutants.
Acquisition of resistance - Gene transfer
Genes encoding resistance to antimicrobial drugs can spread to different strains, species and genera, through conjugative transfer of R plasmids. These often carry several different resistance genes, each encoding resistance to specific antimicrobial drug. When organism received R plasmid, becomes resistant to several meds simultaneously. Some cases resistance genes originate via spontaneous mutation of common bacterial genes, like one encoding target of the drug. Some genes may originated from soil microbes that naturally produce that antibiotic.
Examples of emerging antimicrobial resistance - Enterococci
Group of bacteria that is part of normal intestinal microbiota and common cause for healthcare associated infections. Enterococci are intrinsically less susceptible to manny common antibiotics ex; their penicillin binding proteins have low affinity for certian beta lactam antibiotics and may also have R plasmids. Some strains are resistant to Vancomycin called cancomycin-resistant enterococci VRE, drug thats used as last resort to gram pos organimsims resistant to beta lactam and the strains are encoded on the plasmid.
Broad-spectrum: Ampicillin, Amoxicillin
Has modified side chains that give them broad spectrum of activity. Similar to natural penicillins. Retain activity against penicillin sensitive gram pos and active against gram neg. They can be inactivated by beta lacamases.
Therapeutic index
Ideal antimicrobial drugs are nontoxic to humans, most can be harmful at high concentrations. The toxicity of a given drug is expressed as therapeutic index or the lowest dose toxic to pt divided by dose typically used for therapy. Antimicrobials with high index are less toxic, often cause drug acts against vital biochemical process of microorganisms that not in human. When has low index, concentration in pts blood must be monitored to make sure not reach toxic level. Drugs too toxic can be used for topical applications.
Heavy metals Melarsoprol, Sodium Stibogluconate, Meglumine Antimonate
Inactivate sulfhydrayl groups of parasitic enzymes but very toxic to hosts as well. Melarsoprol used treat Trypanosomiasis, but treatment can be leathal. Sodium stibogluconate and meglumine antimonate used treat leishmaniasis.
Antibacterial Medications that inhibit Cell wall synthesis
Includes B-lactam drugs, Penicillians, Cephalosporins, Carbapenems, Monobactrams, Vancomycin, and Bacitracin. Bacterial cell walls are unique because peptidoglycan. Antimicrobial meds that interfere with synthesis of cell wall do not affect eukaryotic cells and often have high therapeutic index.
Extendedspectrum: Ticarcillin, Piperacillin Penicillins + beta lactamase inhibitor Augmentin (combo of amoxicillin and clavulanic acid)
Increased activity against gram-negative rods, including Pseudomonas species that are uneffected by many antimicrobial drugs. Group less active with gram pos and are destroyed by many beta lactamases. Combo of agents not new drug. Beta lactamase inhibitor interferes with activity of some types of beta lactamases, protecting penicillin against enzymatic destruction.
Allylamines Naftifine, Terbinafine
Inhibit enzyme in pathway of ergosterol synthesis. Administered topically to treat dermatophyte infections. Terbinafine can be taken orally.
Trimethoprim
Inhibits bacterial enzyme that catalyzes metabolic step following one inhibited by sulfonamides. Drug has little effect on enzymes counterpart in human cells. Combo of Trimethoprim and Sulfonamide has synergistic effect called Bactrim, and used to treat UTIs. Resistance is commonly due to a plasmid encoded alternative enzyme with lower affinity to drug; the genes that encode resistance to sulfa drugs are often carried on the same plasmid.
Isoniazid
Inhibits synthesis of mycolic acid, a major component of mycobacterial cell wall
Interfere with Cell wall Synthesis Echinocandins ; Caspofungin
Interfere with beta 1,3 glucan synthesis causing cell to burst. Used to treat Candida infections as well as invasive aspergillosis that resists other treatments.
Azoles Imidazoles and Triazoles
Interfere with ergosterol synthesis, leading to defective cell membranes; active against wide variety of fungi. Used to treat variety of systemic and localized fungal infections. Triazoles less toxic than Imidazoles Imidazoles ; Ketoconazol - Used systemicall, but more severe side effects., Miconazole and Clotrimazole - used in nonprescription creams, ointments and suppositories to treat vaginal yeast infections also applied to skin. Triazoles - Fluconazol and Voriconazol, increasingly used to treat systemic fungal infections
Folate antagonists Pyrimethamine, Sulfonamide
Interfere with folate metabolism; used to treat toxoplasmosis and malaria
Metronidazole (Flagyl)
Interferes with DNA synthesis and function only in anaerobic microorganisms. Selective toxicity is due to fact that anaerobic metabolism is required to convert medication to active form. Active form binds DNA, interfereing with synthesis causing damaging breaks. Used to treat bacterial vaginosis and Clostridium difficil-associated disease.
Examples of emerging antimicrobial resistance - Enterobacteriaceae
Intrinsicall resistant to many antimicrobial meds because outer membrane prevents drugs from entering cell. Situation complicated when enterics developed ability to produce beta lactamase, allowing strains to resist effects of Ampicillin and other penicillins. Some strains developed ability produce extended spectrum beta lactamases ESBLs, making them resistant to most cephalosporins and aztreonam and penicillins. Recently Carbapenem resistant Enterobacteriaceae CRE have been discovered which are resistant to nearly all available antimicrobial drugs, produce enzyme that inactivates carbapenems, group of drugs considered last resort for treating infections caused by ESBL producing bacteria.
Inhibit intestinal protozoa
Iodoquinol, Nitazoxanide, Nitroimidazoles, Quinacrine
Avermectins Ivermectin
Ivermectin causes neuromuscular paralysis in parisites used treat infections by Strongyloides and tissue nematodes
Benzimidazoles Mebendazole, Thiabendazol, Albendazol
Mebendazole bind to tubulin of helminths, blocking microtubule assembly and inhibiting glucose uptake. poorly absorbed in intestine making effective for treating intestinal, but not tissue helminths. Thiabendazole may have similar mechanisim, but well absorbed and many toxic side effects. Albendazole used treat tissue infections caused by Echinococcus and Taenia solium.
Quinolones Chloroquine, Mefloquine, Primaquine, Tafenoquine
Mechanism not clear. Chloroquine concentrated in infeted RBC and drug of choice for preventing or treating RBC stage of malarial paracite. Effects may be due to inhibition of enzyme that protects parasite for toxic by products of hemoglobin degredation. Primaquine and Tafenoquine destroy liver stage of parasite and used to treat relapsing forms of malaria. Mefloquine used treat infection caused by chloroquine resistant strans of malarial parasite
Pyrazinamide
Mechanism unknown
Quinacrine
Mehcanism unknon, but may be due to interference with nucleic acid synthesis
Penetration and uncoating; Fusion or endocytosis
Membrane fusion 1) Absorption; spikes on viron attach to specific host cell receptors 2 Membrane fusion; envelope of virion fuses with plasma membrane 3 Nucleocapside released into cytoplasm; viral envelope remains part of plasma membrane 4 Uncoating ; Nucleic acid separates from capsid with fusion of viron and host cell membrane NAKED VIRUSES CANNOT FUSE Endocytosis 1 Absorption; attachment to receptors triggers endocytosis 2 Endocytosis; Plasma membrane surrounds virion, forming endocytic vesicle. 3 Release from vesicle; envelope of virion fuses with endosomal membrane 4Uncoating; nucleic acid separates from capsid.
Determining susceptibility of bacterial strain to antimicrobial drug - Minimum inhibitory and bactericidal concentrations (MIC and MBC)
Minimum inhibitory concentrations MIC - lowest concentration of specific antimicrobial drug needed to prevent growth of given bacterial strain invitro. Determined by growing test strain in broth cultures containing different concentrations of antimicrobial. Dilutions used to generate decreasing concentrations of drug in tubes containing suitable growth medium. Fixed concentrations of bacteria added and incubated for 16 hours and examine for growth. Lowest concentration of drug that prevents growth is MIC. If strain is inhibited by given concentration of drug does not mean infection caused by strain can be treated with drug. Ex; MIC of 16ug/ml would be considered resistant to drug if level achieved in blood is less than that. Microbes on border between susceptible (treatable) and resistant (untreatable) are intermediate. Minimum bactericidal concentrations MBC - lowest concentration of specific antimicrobial drug that kills 99.9 cells of given bacterial strain in vitro. Determined by finding how many live organisms remain in tubes from MIC test that showed no growth. Small sample from tubes transferred to plate containing antibiotic free agar medium. If sample not make colonies then no cells survived that drug concentration making it bactericidal. If colonies form, they are counted. MIC and MBC give precise info regarding organisms susceptibility but is labor intense and expensive.
Release
Most via budding viral proteins spikes insert into host cell membrane;matrix proteins accumulate; nucleocapsid extrunted Covered with matrix protein and lipid envelope Some obtain envelope from organelles Naked viruses released when host cell dies, often by apoptosis intitiated by virus or host
Slowing emergency and spread of antimicrobial resistance - Responsibilities of physicians and heathcare workers.
Need to inc efforts to idendify cause of infection and if appropriate prescribe antimicrobials. Need to educate pts about proper use to inc compliance. Expensive in short term, save lives and money later
Nitazoxanide
New drug used to treat cryptosporidiosis and giardiasis
Adverse effects ; Suppression of Normal Microbiota
Norm microbiota helps in host defense by excluding pathogens. When altered, pathogens normally unable to multiply can to high numbers. Pts with broad spectrum antibiotics orally can develop antibiotic associated colitis, caused by toxin producing strains of Clostridium difficile.
Viron aka viral partical
Nucleic acid, protein coat. Protein coat is capside which protects nucleic acids. Carries required enzymes and composed of identical subunites called capsomers. Capside plus nucleic acids called Nucleocapsid. Enveloped viruses- have lipid bilayer envelope. Matrix protein - Between nucleocapside and envelope Naked viruses - lack envelope; more resistant to disinfectants
Interfer with Nucleic Acid synthesis
Nucleoside analogs, Non-nucleoside polymerase inhibitors, Non-nucleoside reverse transcriptase inhibitors Take advantage of error prone, virally encoded enzymes that replicate viral nucleic acid. Drugs generally limited to treating infections caused by herpesviruses or HIV
Commercial modifications of antimicrobial susceptibility testing
Offer advantages like less labor intense, One test has results in 4 hours. Uses cards with wells containing specific antimicrobial concentrations. Highly automated system inoculates and incubates cards, determines growth rate by reading turbidity and uses math to interpret results and determine MIC to 6-15 hrs. Another test, E test, is mod of disc diffusion test, using strip containing gradient of concentrations of antimicrobial drug. Multi strips, each with diff drug, placed on surface of agar medium that has been uniformly inoculated with test organism. Incubation test organism grows, zone of inhibition form around strip. Cause gradient of drug concentrations, zone of inhibition is teardrop shaped . MIC determined by reading printed number at point where bacterial growth intersects strip.
Natural penicillins; penicillin G, penicillin V
Original penicillins produced naturally by mold Penicillium chrysogenum, are narrow spectrum antibiotics. Active against gram positive and a few gram negative bacteria. Penicillin G is destroyed by stomach acid, and so it usually must be administered by injection. Penicillin V can be taken orally because is more stable in acid. Bacteria that produce penicillinase are resistant to natural penicillins
Slowing emergency and spread of antimicrobial resistance - Global impacts of use of antimicobial drugs
Overuse is world wide concern. Countires may vary in laws and customes. Organism develops resistance in one country can spread. In developing countries, antimicrobial drugs availabe on nonprescription basis. Many people believe that OTC availability drugs should be restricted or eliminated. Antibiotics in animal feed, low levels of drugs in feed inc growth animals, but this selects for drug resistant organisms. Infections caused by drug resistant Salmonell strains have been linked to animals whose feed was supplemented with drugs.
Adverse effects ; Allergic reactions
People develop allergies to antimicrobial drugs. Allergys result in fever, rash, or anaphylactic shock. People who have allergic reactions to antimicrobial meds must alert doctor so alternative drug can be prescribed.
Piperazines Piperazine, Diethylcarbamazine
Piperazine causes flaccid paralysis in worms and can treat Ascaris infections. Diethylcarbamazine immobalizes filarial worms and alters surface, which enhances killing by immune system. Resulting inflammatory response can cause tissue damage
Mechanisms of acquired resistance -dec uptake of drug
Porin proteins in outer membrane of gram neg bacteria selectively permit small hydrophobic molecules to enter cell, changes therefore can prevent drugs from entering cell stopping entry of drug, organism avoids effects
Acquired Resistance
Previously sensitive organisms develop acquired resistance through spontaneous mutation or horizontal gene transfer.
Nucleoside Anaogs Acyclovir, Ganciclovir, Ribavirin, Zidovudine, Didanosine, Lamivudine
Primarily used to treat infections caused by herpesvirus or HIV; do not cure latent infections. Drugs are converted winthin eukaryotic cells to nucleotide analog aka are nucleoside analogs or compounds similar in structure to nucleoside and phosphorilated to form nucleotide analog; virally encoded enzymes prone to incorporate these than host, resulting in premature termination of synthesis or improper base pairing of viral nucleic acid. Only effective against replicating viruses. Acyclovir used to treat herpes simplex virus HSV and varicella zoster virus VZV infections. Granciclovir used to treat cytomegalovirus infections in immunocompromised pts. Ribavirin used treat respiratory syncytial virus RSV infections in newborns. Nucleoside analogs used to treat HIV infection interfere with activity of reverse transcriptase and are called nucleoside reverse transcriptase inhibitors NRTIs. Combinations of nucleoside analogs such as Zidovudine, Didanosine, Lamivudine used treat HIV infections.
Non-nucleoside polymerase inhibitors Foscarnt
Primarily used to treat infections caused by herpesviruses They inhibit activity of viral polymerases by binding to site other than nucleotide binding site. Foscarnet used to treat Granciclovir resistant cytomegalovirus CMV and acyclovir resistant herpies simplex virus HSV
Prevent Assembly and release of viral particles
Protease inhibitors, Neuraminidase inhibitors
Assembly
Protein capsid forms;genome,enzymes packaged takes place in nucleus or in organelles of cytoplasm
Tetrahydropyrimidines Pyrantel pamoate, Oxantel
Pyrantel pamoate interferes with neuromuscular activity of worms, causing type of paralysis. It is not readily absorbed from gastrointestinal tract and is active against intestinal worms including pinworm, hookworm, and Ascaris. Oxantel can be used to treat Trichuris infections.
Interfereing with Viral Uncoating Amantadine and Rimantadine
Reduce severity and duration of influenza A infections, but resistance limits use. After virus enters host cell, nucleic acid must separate from the protein coat for replication to occur so drugs that interfere with uncoating step prevent viral replication.
Replication of RNA virus
Replication of RNA viruses Majority single stranded replicate in cytoplasm require virally encoded RNA polymerase (replicase) that lacks proofreading, allowing antigenic drift SS+rna used mRNA ss-RNA, dsRNA viruses carry replicase to synthesize + strand Some RNA viruses segmented; reassortment resuts in antigenic shift
Monobactams; Aztreonam
Resistant to beta lactamases but susceptible to extended spectrum beta lactamases; can be given to patients who are allergic to penicillin because diff structure. Primarily active against members of the family Enterobacteriaceae aka are gram neg rods.
Carbapenems Imipenem, Meropenem, Ertapenem, Doripenem
Resistant to inactivation by beta lactamases, but susceptible to carbapenemases and are effective against gram neg and pos. Imipenem must be given in combination with a drug that inhibits certain kidney enzymes in order to avoid its inactivation
Glycylcyclines: Tigecycline
Reversably bind to 30S ribosomal subunit, blocking attachment of tRNA and preventing translation, but have wider spectrum of activity.Bacteriostatic against many gram pos and gram neg bacteria. Effective against many bacteria that have acquired resistance to Tetracyclines.
Tetracyclines Tetracycline, Doxycycline, Glycylcyclines
Reversably bind to 30S ribosomal subunit, blocking attachment of tRNA and preventing translation. Drugs are actively transported into prokaryotic cells, but not animal cells, which concentrates them on bacteria. Bacteriostatic against some gram pos and gram neg bacteria; Resistance is generally due to decreased accumulation, either through decreased uptake or increased excretion.
Penicillinase-resistant penicillins Methicillin, Dicloxacillin
Scientists developed these in response to penicillinase. Similar to natural penicillins, but resistant to inactivation by the penicillinase of staphylococci. Some penicillinase producing bacteria acquire ability to make altered PBP to which beta lactam drugs no longer bind. Staphylococcus aureus stains that do this are called MRSA or methicillin resistant S. aureus
Pyrazinoisoquinolines Praziquantel
Single does of praziquantel is effective in eliminating wide variety of trematodes and cestodes. It is taken up but not metabolized by the worm, ultimately causing sustained contractions of worm.
Adverse effects ; Toxic Effects
Some antimicrobials are toxic at high concentrations or occationally cause adverse reactions. Aminoclycosides like streptomycin can cause kidney damage, impair balance, or irreversible deafness. Pts taking these drugs must be monitored because of low therapeutic index. Some have severe effects and are reserved for only life threatening conditions. Chloramphenicol causes aplastic anemia or when body is unable make WBC and RBC.
Mechanisms of acquired resistance - Drug inactivating enzymes
Some bacteria produce enzymes that chemically modify specific drug, interfering with function. ex penicillinase destroys penicillin. Another enzyme chloramphenicol acetyltransferase, chemically alters antibiotic chloramphenicol making inefective
Beta-lactamase
Some bacteria resist effects of certain beta lactam drugs by synthesizing beta lactamase, an enzyme that breaks critical beta lactam ring, destroying activity of antibiotic. There are various beta lactamases and they differ in range of drugs they destroy. Penicillinase is a beta lactamase that inactivates only members of penicillin family. Extended spectrum beta lactamases ESBLs inactivate wide variaty including penicillins and cephalosprins. Gram neg bacteria produce more extensive array of beta lactamase than gram pos.
Antibacterial medications that interfere with metabolic pathways aka Folate Biosynthesis
Sulfonamides, Trimethoprim Each inhibit different steps in pathway that leads initially to synthesis of folate and to synthesis of coenzyme required for nucleotide biosynthesis. Animal cells lack enzyme in folate synth portion of pathway aka why folic acid is dietary requirement.
Malarone
Synergisit combo of Atovaquone and Proguanil Hydrochloride used to treat malaria. Atovaquone interferes with mitochondrial electron transport while Proguanil disrupts folate synthesis. Combo is active against both blood stage and early liver stgbe of Plasmodium species.
Interfere with Nucleic acid synthesis Flucytosine
Synthetic derivative of cytosine, one nucleobases. Used to treat systemic yeast infections; enzymes within yeast cells convert drug to 5-florouracil, which inhibits enzyme required for nucleic acid synthesis, not effective against most molds; resistant mutants are common. Usd in combo with Amphotericin B or alternative drug.
Interfere with plasma membrane synthesis and function Polyenes, Azoles, Allylamines
Target of most antifungal drugs is ergosterol, sterol found in plasma membrane of fungal, not human cells
Examples of emerging antimicrobial resistance - Streptococcus pneumoniae
Until recently was leading cause of pneumonia in adults but remained sensitive to penicillin. Some isolates now resistant to drug. This acquired resistance is due not to production of beta lactamase, but changes in chromosomal genes coding for targets of penicillin-PBPs. Modified targets have lower affinities for drug. Nucleotide changes do not appear to come from point mutations as one might expect, due to acquisition of chromosomal DNA from other species of Streptococcus. S. pneumoniae can aquire DNA through DNA mediated transformation.
Conventional disc diffusion method - Kirby-Bauer disc diffusion test
Used to determine susceptibility of bacterial strain to battery of antimicrobial drugs. Standard concentration of strain first spread on surface of agar plate. 12 discs, each have known amount of diff drug, placed on surface of medium. During incubation, various drugs diffuse outward from discs forming concentration gradient around disc. Bacteria grow, forming film of growth on plate, except regions around discs where bacteria killed or growth inhibited. Clear zone around disc reflects degree of susceptibility of organism to drug. Zone size influenced by characteristics of drug like molecular weight and stability and amount in disc. Charts used correlating size of zone of inhibition to susceptibility of bacteria to drug.
Prevent Genome Integration Raltegravir
Used to treat HIV infections. Inhibit HIV encoded enzme integrase, preventing virus from inserting DNA copy of genome into host cell.
Non-nucleoside reverse transcriptase inhibitors NNRTI Nevirapine, Delavirdine, Efavirenz
Used to treat HIV infections. Inhibit activity of reverse transcriptase by binding to site other than nucleotide binding site and often used in combo with nucleoside analogs
Prevent viral entry Enfuvirtide, Maraviroc
Used to treat HIV infections. Prevent virus from entering host cells. Enfuvirtide does by binding to HIV protein that promotes fusion of viral envelope with cell membrane. Maraviroc blocks HIV co-receptor CCR5.
Protease inhibitors Indinavir, Ritonavir, Saquinavir, Nelfinavir
Used to treat HIV. Inhibit protease, essential enzyme of HIV by producing infectious viral particles, by binding to active site. When HIV replicates, several proteins translated as polyprotein, single AA chain that must be cleaved by protease to release individual proteins.
Eflornithine
Used to treat infections caused by some types of Trypanosoma; inhibits enzyme ornithine decarboxylase
Interfere with cell division Griseofulvin
Used to treat skin and nail infections. Interferes with action of tubulin, structure required for nuclear division. Because tubulin part of eukarytoci cells, slelective toxicity of drug may be due to greater uptake by fungal cells.Taken orally for months; concentrates in dead karatinized layer of skin; taken up by fungi invading cells and inhibits division. Active only against fungi that invade keratinzied cells. Because
Nitrofurtimox
Used treat acute Chagas disease; forms reactve oxygen radicals that toxic to parasite and host
Replication of DNA viruses
Usually in nucleus Poxviruses are exception; replicate in cytoplasm, encode alll enzymoes for DNA, RNA synthesis dsDNA replication straightforward ssDNA similar except complement first synthesized ds+- DNA to ss + RNA to protein -ss-DNA to ds +-DNA to ss+RNA to protein ss+DNA to ds+-DNA to ss+RNA to protein
Gen characteristics of Virus
Viral genome have either DNA or RNA, not both. Genome linear or cirular, double or single stranded which affects replication strategy. Viruses have protein components for attachment. Phages have tail fibers, animal viruses have spikes, which allow viron to attach to specific receptor sites. Generally come in 3 shapes, Icosahedral, helical, complex.
Process of budding
Viral proteins that will become envelope spikes insert into host plasma membrane Viral matrix protein coats inside of plasma membrane Nucleocapsid extrudes from host cell, becoming coated with matrix proteins and envelope with protein spkes New virus is released
Animal virus replication' 5 step infection cycle; Attachment
Viruses binde to receptors Usually glycoproteins on plasma membrne Often more than one required ex; HIV binds to 2 Normal function unrelated to viral infection Specific receptors required; limits range of virus ex; dogs do not contract measles from humans
Mechanisms of action of antiviral drugs
Viruses rely almost exclusively on host cells metabolic machinery for their replication, making difficult to target for selective toxicity. No cell wall, ribosomes, or any other structure targeted by antibiotics. Many encode own polymerase which is potential target for antiviral drugs or drugs that interfere with viral replication.
Key cahracteristics include
genome structure (nucleic acid and strandedness), and host infected. Other characterisitcs are viral shape, disease symptoms
