Antimicrobial Agents, Resistance, and Susceptibility Testing
What you must know
Antimicrobial questions test mechanisms of action (cell wall synthesis inhibitors - beta-lactams, glycopeptides; protein synthesis inhibitors - aminoglycosides, tetracyclines, macrolides; nucleic acid inhibitors - fluoroquinolones; folate pathway inhibitors - sulfonamides), spectrum, pharmacokinetics, and resistance mechanisms (enzymatic inactivation, target modification, efflux, reduced permeability). Understand intrinsic vs. acquired resistance (e.g., Mycoplasma lacks a cell wall and is intrinsically beta-lactam resistant), MIC and breakpoints, disk diffusion (Kirby-Bauer) and broth dilution, and antibiogram interpretation. Know the concept of withdrawal times and prudent use in food animals.
Traps
Questions love intrinsic resistance (Mycoplasma to beta-lactams, Gram-negative outer membrane limiting some drugs) versus acquired resistance (plasmid-mediated beta-lactamase). Another trap is confusing bactericidal vs. bacteriostatic with spectrum. Watch for questions that assume a larger zone of inhibition always means a better drug - zone size must be interpreted against species-specific breakpoints. Also beware of conflating MIC with therapeutic outcome without considering pharmacokinetics.
Rule of thumb
If the stem describes resistance that is predictable for the whole species regardless of prior exposure, it is intrinsic; if it emerges after exposure and is transferable, it is acquired.
Worked example
A question asks why beta-lactams fail against Mycoplasma. The answer is intrinsic resistance due to absence of a cell wall, the drug target. The discriminator is mechanism of action versus target absence.