Microbiology · virtual laboratory
Disk diffusion methodKirby-Bauer antibiotic susceptibility test
Work through the whole procedure — prepare and standardise the inoculum, pour and inoculate the plate, apply the disks, incubate, read the zones, measure them yourself and interpret what you measured. Read the theory first or go straight to the bench.
What the test answers
Antimicrobial susceptibility testing exists to answer one question about one patient's isolate: which of these drugs is likely to work? Choosing without it is guesswork, and guesswork both fails patients and selects for resistance.
The Kirby-Bauer disk diffusion method answers it by letting the drug and the organism compete on an agar plate. A paper disk carrying a known amount of antimicrobial is laid on a lawn of the organism. The drug leaves the paper and spreads outwards through the agar, so it is most concentrated at the disk and falls away steadily with distance.
Why a clear circle appears
Two things happen at once under that disk. The drug diffuses outwards, and the bacteria multiply. Close to the disk the concentration stays above the level that inhibits this organism, so no visible growth appears. Further out the concentration has fallen too low before the population became large enough to see, and growth fills in.
The edge of the clear area is where those two curves cross. That is why a zone diameter is a meaningful number at all — and also why it is not a measure of potency. It reflects how fast the molecule moves through agar as much as how susceptible the organism is, which is exactly why a diameter can only be read against criteria built for that organism, that agent and that disk content.
What changes the answer
Inoculum density
More organisms in the drug's path means growth reaches closer to the disk. A heavy suspension shrinks the zones; a light one inflates them.
Agar depth
The drug diffuses in three dimensions. A thin layer lets it spread further sideways and reads falsely susceptible; a thick layer does the reverse.
Lawn uniformity
A confluent lawn gives a sharp circular edge. Patchy growth gives a ragged one that cannot be measured reliably.
Disk spacing
Zones that overlap have no readable edge. Disks stay at least 24 mm apart centre to centre and 15 mm in from the rim.
For educational use only. The plate is driven by a teaching model, and the interpretive criteria shipped with it are configurable teaching values — not a clinical standard. Real susceptibility interpretation depends on the guideline in force, the organism, the antimicrobial agent, the disk content and the testing conditions.