Zone of Inhibition
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
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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Update: June 2012