Microbiology Lab Practice Exam

The zone of inhibition in susceptibility testing reflects how effectively an antibiotic can stop that particular organism under standard conditions. In the disk diffusion method, an antibiotic-soaked disk sits on a plate inoculated with the organism, and the antibiotic diffuses outward, creating a concentration gradient. Where the drug concentration remains above the organism’s inhibitory level, growth is prevented, producing a clear halo around the disk. The size of this halo (the zone) correlates with how susceptible the organism is: larger zones mean greater sensitivity, while small or absent zones indicate resistance or reduced susceptibility. Standards specify what zone sizes define susceptible, intermediate, or resistant for each antibiotic-organism pair, guiding interpretation.

This test doesn’t provide an exact dosage to administer, doesn’t measure bacterial growth rate in rich medium, and doesn’t determine the antibiotic’s molecular weight.