03 Drugs Index

The Mechanisms of Actions of the antibacterial agents:
1. Inhibition of bacterial cell wall (peptidoglycan) synthesis:
a. Bacitracin
b. Glycopeptides: vancomycin & teichoplanin
c. B-Lactam antibiotics: penicillins, cephalosporins, carbapenems (imipenem), monobactams
(Clavulanic acid & sulbactam are B-lactamase inhibitors, but they do not bind all classes of B-lactamase & cannot be depended on to prevent the inactivation of B-lactam antibiotics by all B-lactamases.)

2. Inhibition of bacterial ribosome protein synthesis:
a. Aminoglycosides: gentamicin, kanamycin, tobramycin, streptomycin, netilmicin, neomycin, amikacin
b. Macrolides: erythromycin, clarithromycin, azithromycin, Dynabac (Dirithromycin 250 mg tab)
c. Lincosamides: clindamycin & lincomycin
d. Chloramphenicol
e. Tetracyclines: tetracycline, doxycycline, minocycline
f. Mupirocin: pseudomonic acid

3. Inhibition of bacterial folic acid metabolsim:
a. Sulfonamides: sulfisoxazole, fulfadiazine, sulfamethoxazole
b. Trimethoprim

4. Inhibition of bacterial nucleic acid synthesis or activity:
a. Quinolones: nalidixic acid, norfloxacin, ciprofloxacin, ofloxacin, lomefloxacin, novobiocin
b. Rifampin
c. Nitrofurantoin
d. Metronidazole

5. Alteration of bacterial cell membrane permeability
a. Polymyxins: plymyxin B, colistin, polymyxin E
b. Gramicidin A

The Susceptibility Testing of antimicrobial agents:
1. The qualitative assessment
a. Disk/agar-diffusion method: paper disks containing antibiotics onto an agar surface inoculated with the bacterial strain to be tested.
b. Breakpoint method; broth tubes containing a set concentration of antibiotic.

2. The quantitative assessment
a. MIC (Minimum Inhibitory Concentration): The lowest concentration of antibiotic that inhibits microbial growth.
b. MBC (Minimum Bactericidal Concentration) The minimum concentration of antibiotic required to kill the starting inoculum.

The bacteria are usually considered to be susceptible to a drug if the achievable peak serum concentration exceeds the MIC by at least 4x.

The break point of an antibiotics is the concentration of the antibiotics that separates susceptible from resistant bacteria.

When a majority of the isolates of a given bacterial species are inhibited at concentrations below the breakpoint, the species is within the spectrum of the drug.