Exercise 15: Antibiotics Testing - Kirby Bauer Method

• Why might it be necessary to use the Kirby Bauer Method?

to determine the susceptibility of an organism to an antibiotic

The results are often insufficient for clinical purposes, but this test is simple and inexpensive and has considerable value in medical practices.

Antibiotic resistance in bacteria can arise due to mutations in the genomic DNA from constant exposure and overuse of antibiotics as the following video demonstrates.

• Generally speaking, how do you know which antibiotic(s) are more effective against a bacterium based on the results of the Kirby Bauer test?

In this method, a Mueller-Hinton agar or nutrient agar plate is swabbed with the organism in question and paper discs impregnated with antibiotics are placed on the surface of the agar. During subsequent incubation, the antibiotics diffuse into the agar.

If the antibiotic is effective against the organism, a zone of inhibition will appear around the disc.

A measurement of the diameter of the zone of inhibition is made, and this value is compared to standardized tables that indicate organisms’ susceptibilities to various antibiotics. Just because an antibiotic produces a wide inhibitory zone does not automatically indicate that the organism is sensitive to the antibiotic.

• Accurate measurement is essential for determining the susceptibility category (sensitive, intermediate, resistant).

• Consistency in measurement techniques is important to ensure reliable results across different tests.

• The interpretation of the zone size is guided by established clinical breakpoints for each antibiotic

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Reference Chart to Determine Susceptibility of Select Antibiotics.

Reference Chart to Determine Susceptibility of Select Antibiotics.

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