Antimicrobial Susceptibility Testing
1. Principle
Measure whether an isolate can grow in the presence of defined antimicrobial concentrations and interpret results as Susceptible / Intermediate / Resistant using standardized breakpoints (CLSI/EUCAST).
2. Step-by-Step Procedure (conceptual)
- Obtain a pure culture (Culture and Isolation).
- Prepare standardized inoculum (typically log-phase; 0.5 McFarland).
- Test by disk diffusion, broth microdilution (MIC), gradient strips, or automated systems.
- Incubate under specified conditions.
- Read zones/MICs → interpret with current breakpoints → report with comments (e.g., intrinsic resistance).
3. Interpretation
- S: Likely therapeutic success at standard dosing.
- I / SDD: Uncertain or dose-dependent success.
- R: Likely failure at standard regimens.
- Pitfalls: Wrong inoculum; mixed cultures; missing inducible resistance; breakpoint updates; biofilm/persister physiology ≠ AST.
4. Clinical Use Cases
- Gold standard for: Guiding definitive antibacterial therapy for cultivable bacteria.
- Alternative / adjunct: Molecular resistance markers (PCR, Whole-Genome Sequencing) when culture slow or for epidemiology.
5. Comparison with Other Methods
| Method | Pros | Cons |
|---|---|---|
| Phenotypic AST | Directly measures growth inhibition | Needs pure growth; time |
| Gene PCR | Fast for key genes | Gene ≠ expression; incomplete panels |
| WGS | Broad resistome | Prediction imperfect; cost/time |
6. Mnemonic / Visual Aid
Pure → Standard inoculum → Drug → Breakpoint — skip any step, trust the result less.