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Zone of Inhibition - Test
for Antimicrobial Activity
Test Method Summary:
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A pure culture of
microorganisms is grown overnight, and then diluted to a concentration of about 1 x 106 microorganisms per
milliliter. |
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The diluted microbial
suspension is spread by a sterile swab, evenly, over the
face of a sterile agar plate. |
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The antimicrobial agent
is applied to the center of the agar plate (in a fashion
such that the antimicrobial doesn't spread out from the
center) and incubated. |
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If substantial
antimicrobial activity is present, then a zone of inhibition
appears around the test product. The zone of inhibition is
simply the area on the agar plate that remains free from
microbial growth. |
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The size of the zone of
inhibition is usually related to the level of antimicrobial activity
present in the sample or product - a larger zone of inhibition usually means that the
antimicrobial is more potent. |
Strengths of Zone of
Inhibition Testing:
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Zone of inhibition
testing is fast and inexpensive relative to other laboratory tests for
antimicrobial activity. |
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Zone of inhibition
testing is especially
well suited for determining (albeit qualitatively) the
ability of water-soluble antimicrobials to inhibit the
growth of microorganisms. |
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A number of samples can
be screened for antimicrobial properties quickly using this
test method. |
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A variety of
antimicrobial product types can be tested using this
method. Liquids, coated antimicrobial surfaces, and
antimicrobial-impregnated solid products can all be tested
for their ability to produce a "zone of inhibition." |
Weaknesses of Zone of
Inhibition Testing:
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Zone of Inhibition tests
do not necessarily indicate that microorganisms have
been killed by an antimicrobial product - just that they have been prevented from
growing. |
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Microbial growth agars
themselves may interfere with the function of some antimicrobial agents. |
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The method cannot be used
to test the activity of antimicrobial agents against viruses, since viruses
don't "grow" on agar plates like bacteria (viruses don't
replicate outside of their host organisms). |
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The method has some
natural variability, and zones of microbial inhibition do
not always have clear or regular boundaries. |
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The method is not
classically quantitative (though sometimes the diameter of the zones of
inhibition are measured and recorded). |

Zone of Inhibition Testing is
a fast, qualitative means to measure the ability of an antimicrobial
agent to inhibit the growth of microorganisms.
Antimicrobial Test
Laboratories was built around antimicrobial innovation and
antimicrobial research and development. If your company is
interested in screening chemicals, products, or antimicrobial treatments for their
ability to inhibit the growth of microorganisms, the zone of
inhibition test may be a great place to start.
For more information about
Zone of Inhibition testing,
Contact the Lab Today!
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