1. Drugs, Microbes, Host – The Elements of Chemotherapy
Chapter 12
Drugs, Microbes, Host – The Elements of Chemotherapy

2. Introduction Terminology
Bacteriostatic (inhibits growth) vs. Bactericidal (kills organisms)
Broad spectrum vs. Narrow spectrum
Generic (chemical composition of drug) vs. Trade name (name given to a drug by a manufacture
Sources of antimicrobial agents
Antibiotics - growth products of organisms
Chemosynthetic agents - synthesized in a laboratory
Semi-synthetic agents - growth products of organisms that have been chemically altered in laboratory

3. Colony of Streptomyces, one of nature’s most prolific antibiotic producers

4. Major targets of drugs acting on bacterial cells

5. Mode of action of antimicrobial agents: competitive inhibition
Antibacterial drugs
Involves inhibiting synthesis of a critical metabolite
Bacteriostatic in activity
Examples
Sulfa - competition based on chemical similarity between sulfas and PABA which is needed for folic acid synthesis; folic acid critical in protein and NA synthesis; side effect = possible kidney damage from crystals

6. Mode of action of antimicrobial agents: inhibition of cell wall synthesis
Interfere with one of multiple steps in CW synthesis
Greater effect demonstrated against G(+) than G(-) – most act against synthesis of peptidoglycan; many drugs cannot penetrate LPS
Generally considered to be bactericidal - cause lysis

7. Mode of action of antimicrobial agents: inhibition of cell wall synthesis
Drugs with beta lactam ring - prevent cross-linking in last step of synthesis
Penicillins
Original penicillins effective only against G(+) - esp. staph & strep
Semisynthetic penicillins developed to overcome penicillinase (beta lactamase) producing strains (i.e.methicillin, oxacillin & nafcillin) and broaden spectrum to G(-) (i.e. ampicillin & carbenicillin)
Toxicity involves hypersensitivity

8. Mode of action of antimicrobial agents: inhibition of cell wall synthesis
Cephalosporins
Broad spectrum antibiotics
Used when individuals are allergic to penicillin
Modifications to original drug increased spectrum (generations 1, 2, and 3)
Moxalactams
Other drugs - interfere at other steps
Cycloserine
Bacitracin - used primarily as topical (on the skin - not taken internally because of toxicity)
Vancomycin

9. Mode of action of antimicrobial agents: Inhibition of protein synthesis
Inhibit one of many steps in protein synthesis
Examples
Aminoglycosides - includes streptomycin, kanamycin, tobramycin, gentamicin, neomycin, and amikacin; causes misreading of mRNA; bactericidal; synergistic with penicillins
Tetracyclines - block binding to tRNA; bacteriostatic; broad spectrum
Chloramphenicol - prevent peptide bond formation; bacteriostatic; excellent penetration of CNS (useful for treating meningitis); side effect = aplastic anemia
Erythromycin - antimicrobial spectrum similar to penicillin; used especially in penicillin-allergic
Lincomycin and clindamycin - bacteriostatic; clindamycin useful for anaerobes; may cause pseudomembranous colitis

10. Structure of aminoglycoside, tetracycline, chloramphenicol, and erythromycin

11. Effects of drugs on bacterial cell membrane
Act directly on cell membranes (do not need to enter cell to cause damage)
Bactericidal - produce irreversible damage to membrane permeability
Example = polymyxins and colistin
Highly toxic to kidneys and nerves

12. Antifungal drugs Damage cell membranes
Bind or interfere with ergosterol (unique fungal sterol in membrane)
Examples
Polyenes - nystatin and amphotericin B
Imidazoles - clotrimazole, miconazole (topical) and ketoconazole (systemic)
Inhibition of nucleic acid synthesis
Griseofulvin - interferes with mitosis; selectively binds to keratin in skin, hair & nails; used primarily with fungi classified as dermatophytic
5-fluorocytosine

13. Antiviral drugs
Interference with uptake or uncoating of virus - amantadine (used to prevent influenza A)
Inhibition of nucleic acid synthesis
Ribavirin - effective in vitro against a wide range of viruses; highly toxic
Acyclovir - Herpes-specific (genital herpes, cold sores, chickenpox)
Azidothymidine (AZT) - treatment of HIV infections

15. Antibiotic resistance in bacteria
Sites of resistance
Membrane transport - LPS layer of G(-) prevent entry of many drugs
Targets of antimicrobial agents (e.g. ribosomes)
Presence of antibiotic-destroying enzymes (e.g. beta lactamase)

16. Antibiotic resistance in bacteria
Mechanisms of changing resistance
Mutations - permanent changes in chromosomes; not caused by antibiotics
Acquisition of new genetic information - methods
Transformation = naked DNA
Transduction = via viruses
Conjugation = via sex pili (sexual recombination) – most rapid method
Selective pressures of antimicrobial therapy - use of antibiotics select for bacteria that are resistant - sensitive bacteria are destroyed

17. Antibiotic susceptibility testing
Susceptibility no longer predictable
Variables affecting outcome of therapy
Condition of host (immune status - underlying diseases)
Site of infection (can drugs get to site?)
Properties of antimicrobial agent
Other drugs taken concurrently
Susceptibility of organism to drug

18. The role of antimicrobials in disrupting microbial flora and causing superinfections

19. Broth dilution methods (Minimal Inhibitory Concentration) – Quantitative Method
Procedure
Uses decreasing concentrations of antimicrobial agents prepared in 2-fold dilutions of broth that will support growth of test organism
Standard inoculum is added to broth containing dilutions of antimicrobial agent, incubated overnight and then examined for growth (turbidity)
Lowest concentration of agent that inhibits growth as detected by lack of visible turbidity = Minimal Inhibitory Concentration (MIC)
Susceptibility and resistance determined by break point of drug (highest conc. of drug in the blood that can be achieved with maximal therapy); if MIC is lower than breakpoint, organism = susceptible; if MIC is higher than breakpoint, organism = resistant

20. Minimum Inhibitory Concentration

21. Disk diffusion (Kirby/Bauer) - Qualitative
Advantage - rapid testing of several antibiotics simultaneously
Uses antimicrobial agents incorporated into filter paper disks placed on agar media causing drug to diffuse creating a concentration gradient (conc. highest closest to disk)
Susceptibility/resistance determined by measuring diameter of zone of inhibition around disk and comparing to established zones for each antibiotic
Standardization - Bauer, Kirby, Sherris & Turck (correlated with MIC’s using large numbers strains & regression analysis)

22. Techniques for preparation and interpretation of disc diffusion tests

23. Antimicrobial gradient strip method (E test)
Disk diffusion method that allows determination of MIC in agar
Consists of plastic strip containing gradient of antimicrobial agent along with an interpretive scale
Performed similar to disk diffusion
Organisms grow in elliptical zone of inhibition around strip relative to concentration of antibiotic along its length
MIC determined by reading scale at point where zone of inhibition intersects strip