1. Chapter 20 Antibacterial Agents
Medical Microbiology
Chapter 20
Antibacterial Agents

2. Antimicrobial Agents
Antibacterial agent - chemical that kills or inhibits the growth of microorganisms at a very low concentration
Protosil (sulfanilamide) in 1935
Antimicrobial agents produced by living organisms are called antibiotics
Fleming’s discovery of penicillin

3. Antibiotics Important Properties: Selective toxicity
Broad-spectrum vs. narrow-spectrum drugs

4. Targets of Antimicrobial Drugs
Antibiotics have one of the following targets:
Inhibition of cell wall synthesis
Disruption of the bacterial membrane
Inhibition of protein synthesis
Inhibition of nucleic acid synthesis
Antimetabolites
Figure 20-1

6. Inhibition of cell wall synthesis
Beta-lactams (penicillins, cephalosporins, etc.) inhibit the enzymes required to build peptidoglycan
structure of the cell wall - Barrel analogy
Penicillin inhibits the formation of the peptide links

7. Inhibition of cell wall synthesis
Vancomycin – only effective against Gram +
Isoniazid, ethambutol, and others are only effective against Mycobacteria
Others – Figure 20-1 and Tables 20-2, 20-3, and 20-4

9. Damage to Membranes Damage to Membranes – Polymixin antibiotics
Only used topically because of toxicity

10. Inhibition of protein synthesis
Aminoglycosides – bind to the 30S sub-unit and cause:
Misreading of mRNA
Early termination of translation
Table 20-5
Tetracyclines – bind to the 30S sub-unit and block tRNA binding
Table 20-6

11. Inhibition of protein synthesis
Others Table 20-6 and Figure 20-1

13. Inhibition of nucleic acid synthesis
Quinolones – inhibit DNA replication enzymes (gyrase)
Ciprofloxicin and others
Table 20-7
Rifampin – disrupts transcription
Table 20-6
Figure 20-1

15. Antimetabolites Antimetabolites:
Sulfonamides and trimethoprim – inhibit folic acid synthesis
broad-spectrum effectiveness
Figure 20-1

17. Drug Targets
What would be a problem with drugs used to fight fungal infections?
How about those used to fight viral infections?

18. Antibiotic Resistance
Mechanisms of resistance:
destroy the drug
Beta-lactamase - enzyme that destroys penicillins and other beta-lactams
alter the drug’s target site
increase drug elimination
become impermeable to the drug

19. Antibiotic Resistance
Acquisition of resistance:
spontaneous mutation
transfer of plasmids containing a resistance gene

20. Antibiotic Resistance
Human practices leading to resistance:
overuse of antibiotics
use of antibiotics by immuno-suppressed individuals
Failure of patients to follow a prescribed drug treatment
use of antibiotics in animal feed