Disk Diffusion
One-Sentence Definition
Disk diffusion (Kirby-Bauer) measures antimicrobial susceptibility by placing antibiotic-impregnated paper disks on an inoculated agar plate and reading the diameter of the growth inhibition zone.
Simple Explanation
The drug spreads outward from a paper disk. The bigger the clear ring around it, the more susceptible the organism.
Method
- Prepare a standardized inoculum — 0.5 McFarland turbidity
- Lawn the plate: Mueller-Hinton agar (with blood for fastidious organisms), correct depth
- Apply disks, incubate 16–20 h at 35 ± 2 °C in the correct atmosphere
- Measure zone diameters in mm and interpret against EUCAST or CLSI breakpoints → S / I / R
The details that ruin the result
Inoculum density, agar depth, disk storage and potency, delay before incubation, incubation atmosphere, and reading the zone edge. A heavy inoculum shrinks zones and manufactures false resistance.
Interpretation
- S susceptible, standard dosing
- I susceptible, increased exposure (EUCAST redefinition — not “intermediate/uncertain”)
- R resistant
Zone diameter correlates with MIC but does not measure it. For quantitative needs use MIC Testing or Broth Microdilution.
Special reading rules
- D-test for inducible clindamycin resistance in staphylococci (flattened zone between erythromycin and clindamycin disks)
- Screening disks: cefoxitin for MRSA detection, ESBL confirmation with clavulanate combinations
Advantages and limits
- Cheap, flexible, no instrument, easy to add or drop agents
- Not suitable for slow-growing or fastidious organisms without modified methods; not quantitative; needs trained reading — increasingly automated with Digital Microscopy and Image AI
Related MOCs
Active Recall Questions
- What does a heavy inoculum do to zone sizes and why is that dangerous?
- What does EUCAST’s “I” category actually mean now?
- What is the D-test detecting?