What physical principle explains why a paper disk impregnated with an antibiotic produces a circular zone of inhibition on a lawn of bacteria in the Kirby-Bauer test?

Antibiotic molecules diffuse radially through the agar, creating a concentration gradient; growth is inhibited where local antibiotic concentration exceeds the organism's inhibitory threshold, producing a circular zone whose radius is inversely related to the inhibitory concentration.

Chemical reactions between the disk matrix and agar produce heat that denatures bacterial proteins in a circular pattern around the disk; the larger the heated zone, the more susceptible the bacteria are to thermal inactivation caused by the antibiotic-impregnated disc binder.

Capillary action draws the antibiotic into the agar only along straight lines radiating from the disk, and where this capillary flow is strongest bacteria are inhibited, creating a circular appearance only due to the diffusion of nutrients rather than the antibiotic itself.

Antibiotic molecules are actively transported by bacterial cells away from the disk, and the balance between bacterial uptake and metabolism creates a depletion zone in which bacteria cannot grow because the antibiotic is sequestered by the cells near the disk.