Disc Diffusion Calculators

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The disc diffusion test (Kirby-Bauer method) is the most widely used method for testing bacterial susceptibility to antibiotics in clinical microbiology laboratories. Paper discs impregnated with standardized antibiotic concentrations are placed on a Mueller-Hinton agar plate inoculated with the test bacterium. As the antibiotic diffuses outward from the disc, it inhibits bacterial growth in a circular zone. The diameter of this inhibition zone is measured and compared to CLSI (Clinical and Laboratory Standards Institute) or EUCAST breakpoints to classify the organism as susceptible (S), intermediate (I), or resistant (R).

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Kirby-Bauer Procedure

  1. Prepare a 0.5 McFarland standard bacterial suspension (~1.5 × 10⁸ CFU/mL)
  2. Swab entire Mueller-Hinton agar plate surface uniformly in 3 directions
  3. Apply antibiotic discs with sterile forceps or a disc dispenser, spacing ≥24 mm apart
  4. Incubate at 35 ± 2°C for 16–18 hours (standard) or up to 24 h for slow growers
  5. Measure inhibition zone diameters (including disc) with calipers or a ruler in mm
  6. Compare to CLSI breakpoints to classify S, I (or SDD), or R

Interpreting Inhibition Zones

Larger zone = more susceptible (antibiotic diffuses further before concentration falls below MIC). Zone diameter breakpoints are antibiotic-specific and organism-specific. Example for E. coli with ampicillin: ≥17 mm = Susceptible; 14–16 mm = Intermediate; ≤13 mm = Resistant.

Factors Affecting Zone Size

  • Antibiotic potency and molecular weight (smaller molecules diffuse faster)
  • Inoculum density — use standardized 0.5 McFarland
  • Agar depth — 4 mm standard depth
  • Incubation temperature and time
  • Disc storage — keep refrigerated, warm to room temperature before use

Quality Control

Run QC strains (E. coli ATCC 25922, S. aureus ATCC 25923) with each batch to verify zone sizes fall within expected ranges before reporting patient results.

Glossary

Disc Diffusion (Kirby-Bauer)
A standardized agar-based antibiotic susceptibility test; inhibition zone diameter compared to CLSI breakpoints classifies bacteria as Susceptible (S), Intermediate (I), or Resistant (R).
Zone of Inhibition
The clear circular area around an antibiotic disc where bacterial growth is suppressed; larger diameter indicates greater susceptibility; measured in mm and compared to species-specific breakpoints.
McFarland Standard (0.5)
A turbidity standard for bacterial inoculum preparation: ≈1.5 × 10⁸ CFU/mL; used to standardize disc diffusion and MIC tests to ensure reproducible results across laboratories.

Frequently Asked Questions

A bacterial suspension standardized to 0.5 McFarland (~1.5 × 10⁸ CFU/mL) is swabbed uniformly onto Mueller-Hinton agar. Antibiotic-impregnated paper discs are placed on the plate. During incubation at 35°C for 16–24 hours, antibiotic diffuses outward from the disc in decreasing concentration. Where concentration falls below the MIC, bacteria grow normally. The diameter of the growth-free zone is measured and compared to CLSI breakpoints to classify the organism as Susceptible (S), Intermediate (I), or Resistant (R).

A larger inhibition zone indicates greater susceptibility — the antibiotic diffused further before reaching a concentration too low to inhibit growth. The zone diameter is compared to species- and antibiotic-specific breakpoints from CLSI or EUCAST tables: above the susceptible breakpoint = S (standard dosing is likely effective); in the intermediate range = I or SDD (may respond to high doses); below the resistant breakpoint = R (treatment unlikely to succeed). Zone size correlates with MIC but is not a direct measurement of MIC.

Mueller-Hinton agar (MHA) is the CLSI-recommended medium for disc diffusion because it: has low inhibitor content (low sulfonamide inhibitors, low thymine/thymidine that interfere with sulfonamide and trimethoprim testing); supports growth of most common non-fastidious pathogens; gives reproducible results when made to correct depth (4 mm); and its performance is well-characterized for breakpoint development. Fastidious organisms (S. pneumoniae, H. influenzae) require supplemented MHA (5% sheep blood, HTM medium) and have separate breakpoints.

A McFarland standard is a turbidity reference used to standardize bacterial inoculum density. 0.5 McFarland = BaSO₄ suspension with turbidity equivalent to approximately 1–2 × 10⁸ CFU/mL — the standard for disc diffusion. Too dense an inoculum compresses zones (false resistance); too sparse an inoculum expands zones (false susceptibility). Turbidity is checked visually against a white card with black lines, or measured spectrophotometrically at 625 nm (A₆₂₅ = 0.08–0.10 for 0.5 McFarland).