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Antimicrobial Agents

Antimicrobial Agents. Use when balance tips in favor of invading MO. Antimicrobial Therapy. When balance between MO and host tilts in direction of MO, body’s normal defense cannot prevent or overcome disease Turn to Chemotherapy - treatment of disease with chemical drugs into body.

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Antimicrobial Agents

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  1. Antimicrobial Agents Use when balance tips in favor of invading MO

  2. Antimicrobial Therapy • When balance between MO and host tilts in direction of MO, body’s normal defense cannot prevent or overcome disease • Turn to Chemotherapy - treatment of disease with chemical drugs into body

  3. Chemotherapeutic Agents • Antimicrobial- to treat infectious disease, act within host • Antibiotic - produced naturally by MO (bacteria, fungi) • Synthetic drug – synthesized, made in laboratory

  4. Successful Antimicrobial • Selective toxicity - harm MO not host (all drugs have some side-effects) • No hypersensitivity reaction – does not elicit harmful host immune reaction • Penetrate - gets to site of tissue infection rapidly, retain for adequate time • No resistance - MO not readilyable to counteract it

  5. Activity of Antimicrobial • Easier to find against prokaryote as different from eukaryotic cell • Fungi, protozoan, helminth are eukaryotes; make finding drug with selective toxicity more difficult • Especially difficult to find drug against virus, require host cell to replicate

  6. Spectrum of Antibiotics • Narrow spectrum - affects relatively few kinds of bacteria • Broad spectrum - effective against large number Gram(+) & Gram(-) bacteria • Problem of broad spectrum antibiotic use is NF destroyed, allow certain NF to flourish and cause opportunistic infection • Superinfection - overgrowth of NF due to antibiotic treatment for an initial infection

  7. Action of Antimicrobial • Bacteriocidal - kill bacteria • Bacteriostatic - prevent growth of bacteria; host’s defense of phagocytosis and antibody eliminate bacteria • Different areas in bacteria serve as target for action of antimicrobial: • Cell wall • Ribosome • Plasma membrane • DNA, RNA • Metabolite

  8. Bacterial Cell Wall Cross Linking • Interference with synthesis of bacterial cell wall should not harm host • Bacterial cell wall contain peptidoglycan not found in eukaryotic cell • Many antibiotics prevent synthesis of peptidoglycan by interfering with linkage by peptide cross-bridge

  9. Inhibition of Bacteria Cell Wall Biosynthesis: Lactam Ring • These antibiotics contain beta lactam ring that bind to group of bacterial enzymes called penicillin binding proteins (PBP) • PBP involved in peptidoglycan cell wall synthesis • Binding of PBP prevents peptide cross linking, cell wall weakened, bacteria undergoes lysis

  10. Beta Lactam Ring Antibiotics • Affect cell wall synthesis, only effective on actively growing MO • These antibiotics include: • Penicillin and derivatives (ampicillin, methacillin, oxacillin, amoxacillin, augmentin) • Cephalosporin (cephalothin, cefuroxime, ceftazidime, cefoxitin) • Carbapenem (imipenem) • Monobactam (aztrenam)

  11. Inhibition of Bacterial Cell Wall Biosynthesis: Others • Bacitracin - interferes with synthesis of peptidoglycan by inhibiting recycling of metabolites • Vancomycin - binds to precursors used in cell wall synthesis; interfere with enzymes that incorporate these precursors into growing cell wall

  12. Inhibition of mRNA Translation • Protein synthesis common feature of all cells • Ribosome structure of eukaryote and prokaryote cell differ (80S vs 70S) • Many antimicrobials specifically interfere with mRNA protein synthesis on prokaryotic 70S ribosomes • Some antimicrobials act on 50S subunit of the ribosome, while others act on 30S subunit of ribosome

  13. Inhibition of Bacteria Translation • Chloramphenicol - acts at 50S, inhibit formation of peptide bond • Erythromycin - acts at 50S, prevent translocation movement of ribosome • Tetracycline - acts at 30S, interfere with tRNA attachment • Aminoglycosides (gentamycin, streptomycin) - act at 30S, cause misreading of mRNA

  14. Injury to Bacteria Plasma Membrane • Polypeptide antimicrobials • Polymyxin B • Colistin • Affect permeability of cells • Result in leakage of macromolecules and ions essential for cell survival

  15. Inhibition of Bacteria DNA/RNA Synthesis • Ciprofloxacin (fluoroquinolone) - bind and interfere with DNA gyrase involved in DNA supercoiling • Metronidazole - breaks DNA strand • Rifampin - binds to DNA dependent-RNA polymerase to inhibit mRNA synthesis

  16. Inhibition Bacteria Folate Synthesis • Antimetabolite - closely resemble normal substrate (analogue), competes for enzyme • Both sulfonamide and trimethoprim interfere with folic acid pathway • Often in single pill used in combination drug therapy: Trimethoprim-Sulfamethoxazole (TMP-SMX, Bactrim) • Broad spectrum antimicrobial

  17. Inhibition of Bacteria Enzymatic Activity • Nitrofurantoin - targets synthesis of several bacterial enzymes and proteins; may also directly damage DNA • Isoniazid - structural analogue of vitamin B6; inhibits synthesis of mycolic acid of Mycobacteria cell wall • Ethambutol - inhibits incorporation of mycolic acid into Mycobacteria cell wall

  18. Summary: Bacteria Antimicrobial

  19. Antifungal Drugs • Nystatin and amphotericin B combine with sterols to disrupt fungal plasma membrane • Effective because animal sterols are mostly cholesterol while fungal membranes contain mainly ergosterol against which the drugs target • Ketoconazole (imadazole) - interfere with sterol synthesis • Griseofulvin - binds to keratin on skin, hair, and nails; interferes with mitosis and fungal reproduction

  20. Antiviral Drugs: Nucleoside Analogue • In viral nucleic acid, analogue insert in place of normal nucleoside • Nucleic acid synthesis stops • Nucleoside analogue binds more strongly with viral enzyme than host cell enzyme • Example: acyclovir for herpes virus; also several nucleoside analogues for HIV infection

  21. Other Antiviral Drugs • Interferon – protein made by host cell for first line of antiviral defense; cloned by recombinant DNA technology, treatment for severe and chronic virus infections • Tamiflu, Relenza – interfere with release of influenza virus from host cell • Protease inhibitors – interfere with proteolytic cleavage of HIV polyproteins into individual proteins, stops replication process • Anti-sense or siRNA (small, interfering RNA) – experimental antiviral drugs, inhibits mRNA translation

  22. Antimicrobial Susceptibility Testing • Important as different MO species and strains have different degree of susceptibility to different antimicrobials • Susceptibility of MO to antimicrobial may change with time, even during course of antimicrobial therapy

  23. Drug Sensitivity Test:Diffusion Test • Kirby-Bauer Test – standardized lab test with antibiotic impregnated disk, diffuses out in a concentration gradient, measure zone of inhibited bacterial growth • E Test – utilizes plastic coated strip containing gradient of antibiotic that diffuses out, allows estimate Minimal Inhibitory concentration (MIC) that prevents visible bacterial growth

  24. Drug Sensitivity Test:Test Tube Dilution • Broth Dilution Test – measures more accurately serial antibiotic dilutions in broth test tube for MIC, followed by plating for Minimal Bacteriocidal Concentration (MBC)

  25. Antibiotic Resistance • Presently a common occurrence • Bacterial drug resistance requires interruption or disturbance of the steps for antimicrobial action

  26. Antibiotic Resistance • Intrinsic resistance - normal genetic, structural, or physiologic state of MO; considered natural and inherited characteristic associated with majority of strains of bacterial group • Acquired resistance - altered cellular physiology and structure caused by changes in a MO genetic makeup; may be a trait associated with only some strains of bacterial group

  27. Acquired Antibiotic Resistance • Acquisition of genes from other MOs via gene transfer mechanisms (i.e., resistance plasmids) • A combination of mutational and gene transfer events

  28. Pathways of Antibiotic Resistance

  29. Enzymatic Degradation: Penicillinase • Resistance to penicillin and other beta-lactam antibiotics • Production of an enzyme that breaks beta-lactam ring

  30. Gram(+) MO Resistance to Beta-lactam Antibiotics • Enzymatic degradation – MO produces beta-lactamase, cleaves ring structure of antibiotic • Altered antimicrobial target – MO mutation of penicllin binding proteins (PBP) so antibiotic no longer binds to it

  31. Gram(-) Resistance to Beta-lactam Antibiotics • Decrease uptake - of antibiotic • Enzymatic degradation – of antibiotic • Altered antimicrobial target - PBP

  32. Dissemination of Antimicrobial Resistance

  33. Prevention of Bacterial Antimicrobial Resistance • Use antimicrobial drugs only when necessary • Finish prescribed course of antimicrobial • Use drugs in combination; microbe less likely to develop resistance to two drugs at the same time: • Consider synergistic effects • Consider antagonistic effects

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