Zone of Inhibition

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Zone of Inhibition

• Bacteria growing in petri dishes

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In the center of the plate is a colony of
Penicillium notatum, a mold that produces
penicillin.
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To test whether an antibacterial agent works, we
will soak a disk filter, like the one shown, in
an antibiotic, soap, or whatever agent we are
testing. The plate is prepared with bacteria and
then incubated overnight.
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In this example, the plate was overlaid with a
bacterial culture of Micrococcus luteus which
forms a yellow "lawn" of growth.
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A zone of inhibition of bacterial growth
surrounds the fungal colony where penicillin has
diffused into the medium.
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Measure Zone of inhibition
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In this figure, the zones of inhibition are
around the filter paper disks. The paper disks
are saturated with an anti-microbial compound.
Bacterial colonies are not growing in these clear
zones.
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The larger the zone, the more effective the
antibacterial agent is.