Question
Antimicrobial susceptibility testing methods – types with respective principle, interpretation, purpose of selective reporting. (4+3+1)
Answer
Antimicrobial susceptibility testing methods — types, principle, and interpretation
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Disc diffusion (Kirby-Bauer) method — Principle: a standardized bacterial suspension (0.5 McFarland) is spread on Mueller-Hinton agar; antibiotic-impregnated discs are placed on the surface; the antibiotic diffuses radially, creating a concentration gradient, inhibiting bacterial growth wherever the local concentration exceeds the organism’s susceptibility threshold, producing a circular zone of inhibition. Interpretation: the zone diameter is measured and compared against standardized breakpoints (e.g., CLSI) to categorize the isolate as Sensitive, Intermediate, or Resistant.
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Minimum Inhibitory Concentration (MIC) determination — Principle: serial doubling dilutions of an antibiotic are prepared in broth (broth macro/microdilution) or agar (agar dilution), each inoculated with a standardized organism suspension; the lowest concentration showing no visible growth is the MIC. Interpretation: the MIC value is compared against defined breakpoints for that antibiotic-organism combination to categorize susceptibility, and also provides a precise numerical value useful for dosing decisions in serious infections.
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E-test (Epsilometer test) — Principle: a plastic strip with a continuous, predefined antibiotic concentration gradient is placed on an inoculated agar plate (similar to disc diffusion); an elliptical zone of inhibition forms, intersecting the strip at a point that directly reads as the MIC. Interpretation: the MIC value read from the strip is compared against standard breakpoints.
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Automated systems (e.g., VITEK, BD Phoenix) — Principle: automated broth microdilution-based systems providing rapid MIC determination and simultaneous organism identification. Interpretation: software-generated categorical result (S/I/R) based on integrated breakpoint databases.
Purpose of selective reporting Selective (cascade) reporting means that only a curated subset of clinically appropriate antibiotics is reported to the treating clinician for a given organism, rather than the full panel tested — reserving broader-spectrum or reserve antibiotics (e.g., carbapenems, vancomycin) from routine reporting unless the organism is resistant to first-line agents. This practice promotes antibiotic stewardship by guiding clinicians toward narrower-spectrum, appropriate first-line therapy, reduces unnecessary/inappropriate use of broad-spectrum and reserve antibiotics (helping preserve their effectiveness and slow the emergence of resistance), and simplifies clinical decision-making by presenting a focused, relevant set of options rather than an overwhelming full susceptibility panel.