Kirby-Bauer Test Calculators

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The Kirby-Bauer test, also called the disc diffusion susceptibility test, is the standard method for testing bacterial sensitivity to antibiotics in clinical microbiology. Antibiotic-impregnated paper discs are placed on agar plates inoculated with a standardized bacterial suspension. During incubation, antibiotics diffuse outward from the discs and inhibit susceptible bacteria, creating clear zones (zones of inhibition). The diameter of each zone is measured and compared to CLSI or EUCAST breakpoints to classify the organism as Susceptible (S), Intermediate (I), or Resistant (R). Results guide antibiotic prescribing decisions in clinical settings.

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Standard Procedure

  1. Prepare 0.5 McFarland standard bacterial suspension (~1.5 × 10⁸ CFU/mL)
  2. Inoculate the entire Mueller-Hinton agar (MHA) plate surface by swabbing in three directions
  3. Allow to dry 3–5 minutes
  4. Apply antibiotic discs with forceps or dispenser — ≥ 24 mm apart, ≥ 15 mm from edge
  5. Incubate at 35 ± 2°C for 16–18 hours (aerobic atmosphere)
  6. Measure inhibition zone diameters in mm with calipers (include disc diameter)
  7. Compare to current CLSI M100 breakpoints; report S, I (SDD), or R

Measuring Zone Diameters

Measure the complete inhibition zone including the disc (6 mm) using calipers or a ruler on the agar back surface (for translucent media). For MRSA with oxacillin: use transmitted light. For trimethoprim-sulfamethoxazole: ignore haze of tiny colonies within the zone. Record diameter in whole millimeters.

Common Antibiotic Disc Concentrations

  • Ampicillin: 10 μg
  • Amoxicillin-clavulanate: 20/10 μg
  • Ciprofloxacin: 5 μg
  • Vancomycin: 30 μg
  • Oxacillin: 1 μg (MRSA detection)

Quality Control

Run QC organisms with every batch: E. coli ATCC 25922; S. aureus ATCC 25923; P. aeruginosa ATCC 27853. Zone diameters must fall within published acceptable ranges before patient results are reported. Out-of-range QC requires investigation before releasing results.

Glossary

Kirby-Bauer Disc Diffusion
Standard antibiotic susceptibility test using impregnated paper discs on Mueller-Hinton agar; inhibition zone diameters compared to CLSI breakpoints to classify bacteria as S, I, or R.
0.5 McFarland Standard
The turbidity reference for bacterial inoculum preparation: ~1.5 × 10⁸ CFU/mL; critical for reproducible disc diffusion results; matched visually or by spectrophotometry (A₆₂₅ = 0.08–0.10).
CLSI Breakpoints
Zone diameter thresholds published by the Clinical and Laboratory Standards Institute (CLSI M100) separating Susceptible (S), Intermediate (I/SDD), and Resistant (R) interpretive categories; updated annually.

Frequently Asked Questions

Standardize bacterial suspension to 0.5 McFarland turbidity (~1.5 × 10⁸ CFU/mL). Swab uniformly over the entire Mueller-Hinton agar surface in three directions. Allow to dry 3–5 minutes. Apply antibiotic discs (at least 24 mm apart). Incubate at 35±2°C for 16–18 hours. Read and measure all inhibition zone diameters in mm. Compare to current CLSI M100 or EUCAST breakpoints for each organism-antibiotic combination. Report S (susceptible), I or SDD (susceptible-dose-dependent), or R (resistant).

The inhibition zone is the clear halo around an antibiotic disc where bacteria cannot grow. Larger zone = more susceptible (antibiotic diffuses further before falling below the MIC). Smaller zone = more resistant. Each antibiotic and organism combination has specific CLSI breakpoints — the zone diameter thresholds separating S, I, and R. Zones falling exactly at the breakpoint (intermediate range) may benefit from higher antibiotic doses or combination therapy. Zone sizes do not directly equal MIC — they are correlated but not interchangeable.

Mueller-Hinton agar (MHA) is the CLSI-specified medium because: it supports growth of most common non-fastidious pathogens; has low inhibitor content (low sulfonamide and trimethoprim inhibitors, low thymidine that interferes with TMP-SMX testing); gives reproducible results at standardized depth (4 mm); and its composition is well-characterized for breakpoint development. Fastidious organisms (S. pneumoniae, H. influenzae) require supplemented MHA (5% sheep blood or HTM medium) and use separate species-specific breakpoints.

The 0.5 McFarland standard (A₆₂₅ = 0.08–0.10) corresponds to ~1–2 × 10⁸ CFU/mL — the critical inoculum density for Kirby-Bauer. Too dense an inoculum (too turbid): bacterial growth compresses inhibition zones, giving falsely smaller zones and potentially misclassifying susceptible organisms as intermediate or resistant. Too light an inoculum: zones are artificially expanded, potentially misclassifying resistant organisms as susceptible. Inoculum density is the single most important variable affecting zone size reliability and should be confirmed visually or with a nephelometer before each test.