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Name: Kristin
Status: Student
Age: 17
Location: N/A
Country: N/A
Date: September 2004

What is a zone of inhibition on an agar plate?

This is an area around a paper disk or colony of bacteria or mold where no other organisms are growing. If you are testing antibiotic sensitivity for example, you can impregnate paper disks with antibiotic and then put them on an agar plate of growing bacteria. The antibiotic then diffuses into the agar away from the disk. If the bacteria are sensitive to the antibiotic, they will not grow near the disk. The size of the zone is proportional to how sensitive the organism is. If the organism is resistant to the antibiotic, it will grow right up to the disk.


It is a circular zone around a disc containing an antibiotic, for example, in which the growth of bacteria susceptible to the antibiotic is inhibited. Typically several million bacterial cells are spread on the agar plate, and if their growth is inhibited, a clear "zone of inhibition" is observed around the antibiotic impregnated disc. If the bacteria are resistant to the antibiotic, a confluent "lawn" of growth (opaqueness) is observed.

Ron Baker, Ph.D

Dear Kristin,

I assume your question refers to an agar plate that has some microbe growing on the plate. In these experiments, one starts with the microbe distributed across the entire plate. If nothing is added to the plate, each microbe will grow, and produce a new microbe by cell division. After this happens for a while (1-3 days depending on the temperature), the agar will get a cloudy appearance because of all of the microbial growth and cell divisions. If no growth occured in a small area of agar, this area would remain clear. That should be sufficient for you to answer the question.

Dr. Jim Tokuhisa

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