Control of Microorganisms: Disinfection, Antibiotics and Antimicrobial Resistance
Introduction
This domain covers chemical and physical control, antibiotic targets, susceptibility testing, and the mechanisms and spread of antimicrobial resistance. The exam tests correct use of terms and interpretation of susceptibility results.
Core rules
- Disinfectants are used on inanimate objects; antiseptics are used on skin or mucous membranes; sterilants kill spores.
- Antibiotics act on cell wall synthesis, protein synthesis, nucleic acid synthesis, folate synthesis, or membrane integrity.
- Resistance mechanisms include enzymatic inactivation, efflux pumps, target modification, reduced permeability, and biofilm formation.
- Disk diffusion and minimum inhibitory concentration tests guide therapy but must use standard methods and organism-specific breakpoints.
- Antimicrobial stewardship means prescribing only when indicated, choosing the narrowest effective agent, and completing the course.
Worked example
A hypothetical drug has breakpoints susceptible ≤1 µg/mL, intermediate 2 µg/mL, and resistant ≥4 µg/mL. An isolate has MIC 4 µg/mL. Because the MIC reaches the resistant breakpoint, report it as resistant, not intermediate. Do not average MICs or interpret a zone size without the correct table for that organism and drug.
Traps
A large inhibition zone does not always mean the drug is clinically preferred; potency and breakpoints matter. Disk diffusion is not suitable for all fastidious organisms. Do not use antibiotics for viral infections. Resistance can emerge during therapy, so culture and susceptibility results matter.