Paper I
Question
Name different methods of antimicrobial susceptibility testing
Answer
Methods of antimicrobial susceptibility testing:
- Disc diffusion (Kirby-Bauer) method — antibiotic-impregnated discs placed on an agar plate lawn-inoculated with the test organism; zone of inhibition diameter is measured and compared against standardized breakpoints to categorize the isolate as Sensitive, Intermediate, or Resistant — the most widely used qualitative/categorical method.
- Minimum Inhibitory Concentration (MIC) determination:
- Broth dilution method (macro/micro-dilution) — serial doubling dilutions of antibiotic in broth are inoculated with a standardized organism suspension; the lowest concentration showing no visible growth is the MIC.
- Agar dilution method — similar principle, using agar plates containing different antibiotic concentrations.
- E-test (epsilometer test) — a plastic strip with a continuous antibiotic concentration gradient placed on an inoculated agar plate; an elliptical zone of inhibition intersects the strip at the MIC value, combining the simplicity of disc diffusion with a quantitative MIC readout.
- Automated systems (e.g., VITEK, BD Phoenix) — automated broth microdilution-based systems providing rapid MIC determination and organism identification simultaneously.
- Minimum Bactericidal Concentration (MBC) — determined by subculturing from clear (no-growth) MIC tubes onto antibiotic-free media; the lowest concentration yielding no bacterial growth on subculture is the MBC, distinguishing bactericidal from merely bacteriostatic activity.
Clinical relevance: choice of method depends on the required precision (qualitative categorization vs. exact MIC value, needed e.g. for certain serious infections or slow-growing/fastidious organisms) and available laboratory resources.