Question
Disc diffusion method of antibiotic sensitivity testing
Answer
The disc diffusion method (Kirby-Bauer method) is the most widely used standard technique for routine antibiotic susceptibility testing in clinical microbiology laboratories, determining whether a bacterial isolate is susceptible, intermediate, or resistant to specific antimicrobial agents.
Principle: a bacterial suspension of standardized density (adjusted to match a 0.5 McFarland turbidity standard) is evenly spread (“lawn culture”) over the surface of Mueller-Hinton agar (the standard medium, chosen for its reproducible, well-defined composition and good diffusion characteristics). Filter-paper discs impregnated with fixed, standardized concentrations of different antibiotics are placed on the inoculated agar surface. During incubation (typically 16–18 hours at 35–37°C), the antibiotic diffuses radially outward from the disc into the surrounding agar, creating a concentration gradient (highest near the disc, progressively lower with distance) — bacterial growth is inhibited wherever the local antibiotic concentration exceeds the minimum needed to inhibit that organism, producing a clear circular zone of inhibition around the disc.
Interpretation: the diameter of the zone of inhibition is measured (in mm) and compared against standardized interpretive criteria/breakpoints (published by bodies such as CLSI — Clinical and Laboratory Standards Institute) specific to each antibiotic-organism combination, categorizing the isolate as Sensitive (S), Intermediate (I), or Resistant (R) to that antibiotic. A larger zone generally indicates greater susceptibility (lower minimum inhibitory concentration), though the relationship is qualitative/categorical rather than directly quantitative (for a precise minimum inhibitory concentration value, quantitative methods such as the E-test or broth microdilution are used instead).
Advantages: simple, inexpensive, allows simultaneous testing against multiple antibiotics on a single plate, and does not require specialized equipment. Limitations: qualitative/categorical rather than quantitative (does not directly give a numerical MIC value); not standardized/validated for all organism-antibiotic combinations (particularly slow-growing or fastidious organisms), for which alternative methods (E-test, automated broth microdilution systems) are preferred.