Antimicrobial Agents and Susceptibility Testing
Core concepts
Know mechanism classes: cell wall (beta-lactams, vancomycin), protein synthesis (aminoglycosides, macrolides, tetracyclines, oxazolidinones), nucleic acid (fluoroquinolones, rifampin), folate (sulfonamides, trimethoprim), and membrane (polymyxins, daptomycin). Understand pharmacokinetics/pharmacodynamics: time-dependent killing (beta-lactams, vancomycin) vs. concentration-dependent (aminoglycosides), plus post-antibiotic effect.
Susceptibility testing
Interpret MIC, disk diffusion, and automated systems. Know CLSI breakpoints, inducible resistance (D-test for clindamycin with erythromycin), ESBL and carbapenemase detection, and MRSA/vancomycin-intermediate detection. Understand intrinsic resistance (e.g., Enterococcus to cephalosporins, Klebsiella to ampicillin).
Traps
- Confusing bactericidal vs. bacteriostatic (an important distinction but often not clinically decisive).
- Forgetting that a D-test is needed before reporting clindamycin susceptible in staphylococci.
- Assuming a susceptible result guarantees clinical success - source control and dosing matter.
- Missing that aminoglycosides require extended-interval dosing for concentration-dependent killing.
Worked rule of thumb
For S. aureus with erythromycin resistance and clindamycin susceptibility, perform a D-test; if positive, report clindamycin as resistant to avoid treatment failure from inducible resistance.