Antibiotics and antimicrobial compounds inhibit the growth of microorganisms such as bacteria, fungi and yeasts. A zone of inhibition is the clear area surrounding a sample of an antimicrobial agent that's been deposited on an agar-based culture. Agar is a natural gel that technicians heat, mix with nutrients and, optionally, samples of the microbe of interest and pour into Petri dishes. The technician adds small samples of the antimicrobial substance to predetermined locations on the cooled agar surface. If the microbe hasn't already been mixed in with the agar solution, testers then expose the dish to the microbes under study.
In the Zone
Over the course of hours or days, microbial colonies will grow on the agar surface, forming a translucent or opaque culture that may have one or more colors. If the antimicrobial agent is effective, it will produce a clear ZOI that is free of life. You measure the ZOI with a ruler or caliper. If the perimeter of the ZOI is clearly defined, you should measure its diameter; otherwise, you should measure its radius. Hold the ruler or caliper on the underside of the Petri dish and make a direct reading in millimeters. Include the size of the antimicrobial sample in the measurement. Take readings of multiple zones to obtain an average size.