Agar diffusion test

Agar diffusion test is a method used in microbiology to determine the susceptibility of microorganisms to antibiotics. It is also known as the Kirby-Bauer test, named after the scientists who developed it.

Overview

The agar diffusion test is a simple, inexpensive, and reliable method for testing the sensitivity of bacteria to antibiotics. The test is performed by placing a paper disc impregnated with a specific concentration of an antibiotic onto an agar plate that has been inoculated with the bacteria to be tested. The antibiotic diffuses from the disc into the agar, creating a gradient of antibiotic concentration. The bacteria grow and form a visible lawn on the agar, except in the area around the disc where the concentration of the antibiotic is high enough to inhibit growth. This area of no growth is called the zone of inhibition.

Procedure

The procedure for the agar diffusion test involves several steps. First, a pure culture of the bacteria to be tested is prepared. This culture is then spread evenly over the surface of a Mueller-Hinton agar plate. Antibiotic-impregnated discs are then placed on the surface of the agar. The plate is then incubated at a specific temperature for a specific period of time, usually 24 hours. After incubation, the plate is examined for zones of inhibition around the antibiotic discs. The size of the zone of inhibition is measured and compared to a standard chart to determine whether the bacteria are sensitive, intermediate, or resistant to the antibiotic.

Limitations

While the agar diffusion test is a valuable tool in the field of microbiology, it does have some limitations. The test is not suitable for all types of bacteria, as some bacteria do not grow well on Mueller-Hinton agar. Additionally, the test does not provide information on the minimum inhibitory concentration (MIC) of the antibiotic, which is the lowest concentration of the antibiotic that will inhibit the growth of the bacteria.

See also

• Antimicrobial susceptibility testing
• Disc diffusion test
• Etest

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