1. Use when balance tips in favor of invading MO
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

5. 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 readily able to counteract it

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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)

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. Summary: Bacteria Antimicrobial

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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)

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

27. 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

28. 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

29. Pathways of Antibiotic Resistance

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

31. 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

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

34. Dissemination of Antimicrobial Resistance

35. 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