Reading Disk Diffusion and MIC Results | Visual AI Learning Guide

Start with the disk. The drug diffuses outward, so its concentration is highest at the disk and falls with distance. Growth stops where the concentration is still above the inhibitory threshold, and that boundary is the edge of the clear zone you measure. Now watch the dilution series: each well holds half the drug of the one before it, and the first well that stays clear marks the minimum inhibitory concentration. Notice that when you raise the drug level, the zone widens and the MIC drops. They move in opposite directions because both are just different ways of locating the same inhibitory threshold. The last step is the breakpoint: the same zone or MIC can be called susceptible or resistant depending on which organism and drug you are reading, so always apply the matching table.

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Two standard methods measure the same thing — how well a drug stops the organism from growing — but report it in different units.

In disk diffusion (the Kirby-Bauer method), a standardized inoculum of the isolate is spread evenly across an agar plate, and paper disks impregnated with fixed amounts of antibiotic are placed on the surface. The drug diffuses outward from each disk, creating a concentration gradient that falls with distance. Where the concentration is still high enough to inhibit growth, no colonies appear; where it has fallen below the inhibitory level, growth reaches the disk edge. The result is a circular clear area, the zone of inhibition, whose diameter in millimeters is measured with a ruler across the disk center. A larger zone means the organism was inhibited at a lower drug concentration, which generally indicates greater susceptibility.

Broth microdilution takes a different route to the same question. A standardized organism suspension is added to a series of wells containing twofold dilutions of antibiotic, so each well holds half the concentration of the previous one. After incubation, the wells are examined for turbidity. The lowest concentration at which no visible growth occurs is the minimum inhibitory concentration (MIC), reported in \(\mu g/mL\). The MIC is therefore a concentration, not a distance, and lower values indicate greater susceptibility.

Neither number is interpreted in isolation. Each organism-drug pair has published breakpoints that convert the measured value into a category: susceptible, intermediate, or resistant. Breakpoints differ by species, by drug, and by the site of infection, because they are derived from pharmacokinetic and pharmacodynamic data linking the drug exposure achievable in the patient to the concentration that inhibits the organism. A 16 mm zone around a given disk may be susceptible for one species and resistant for another, so the raw measurement must always be read against the correct breakpoint table.

References

[1]EUCAST Clinical Breakpoints and Guidance Documentseucast.org