results of a Kirby-Bauer disk diffusion antibiotic susceptibility test

The image shows the results of a Kirby-Bauer disk diffusion antibiotic-susceptibility test. Antibiotic-impregnated disks (containing various antibiotics) are placed onto the agar surface, which contains a "lawn" of growing bacteria isolated from a bull shark caught in Louisiana waters. As the bacteria on the lawn grow, the antibiotic diffusing from the disks inhibits its growth varying degrees. As indicated by the varying zones of growth inhibition surrounding each disk, the Gram-negative bacteria isolated from the shark are resistant to some of the antibiotics in the disks at the concentrations tested. The diameter of the inhibition zone identifies the bacteria as "susceptible," "intermediate," or "resistant" to specific concentrations of an antibiotic.

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