Question
Disc diffusion method of antibiotic susceptibility testing
Answer
The disc diffusion method (Kirby-Bauer method) is the most widely used standard technique for routine antibiotic susceptibility testing, determining whether a bacterial isolate is susceptible, intermediate, or resistant to specific antimicrobial agents.
Principle: a bacterial suspension of standardized density (0.5 McFarland turbidity standard) is evenly spread over Mueller-Hinton agar. Filter-paper discs impregnated with fixed, standardized concentrations of different antibiotics are placed on the inoculated agar surface. During incubation (16–18 hours at 35–37°C), the antibiotic diffuses radially outward, creating a concentration gradient — bacterial growth is inhibited wherever the local concentration exceeds the minimum needed to inhibit that organism, producing a clear circular zone of inhibition.
Interpretation: the diameter of the zone of inhibition is measured and compared against standardized breakpoints (e.g., CLSI), categorizing the isolate as Sensitive, Intermediate, or Resistant to that antibiotic.
Advantages: simple, inexpensive, allows testing multiple antibiotics simultaneously. Limitations: qualitative/categorical rather than giving a numerical MIC directly; not standardized for all organism-antibiotic combinations, particularly fastidious/slow-growing organisms, for which E-test or broth microdilution are preferred.