Disk diffusion test

Disk diffusion test, also known as the Kirby-Bauer antibiotic testing, is a method used to determine the susceptibility of a bacterial strain to antibiotics. It is a crucial technique in microbiology for guiding the choice of antibiotics in treatment and understanding bacterial resistance patterns.

Overview

The disk diffusion test involves the placement of antibiotic-impregnated paper disks onto an agar plate where a bacterial isolate has been spread. The plate is then incubated to allow bacterial growth. The principle behind the test is that antibiotics diffuse from the disk into the agar and inhibit bacterial growth in a concentration-dependent manner. The result is a clear area around the disk where bacteria have not grown, known as the zone of inhibition. The size of this zone is measured and compared to standard values to determine whether the bacteria are susceptible, intermediate, or resistant to the antibiotic.

Procedure

1. A pure culture of the bacteria is diluted and swabbed uniformly across the surface of a Mueller-Hinton agar plate, a medium commonly used for antibiotic susceptibility testing.
2. Antibiotic-impregnated disks are placed on the surface of the agar.
3. The agar plate is incubated at 35-37°C for 16-18 hours.
4. The diameter of the zone of inhibition around each disk is measured in millimeters.

Interpretation

The interpretation of the disk diffusion test is based on guidelines provided by organizations such as the Clinical and Laboratory Standards Institute (CLSI) or the European Committee on Antimicrobial Susceptibility Testing (EUCAST). These guidelines include standardized zone diameter breakpoints for categorizing bacterial isolates as susceptible, intermediate, or resistant to the antibiotics tested.

Advantages and Limitations

The disk diffusion test is widely used due to its simplicity, cost-effectiveness, and the ability to test multiple antibiotics simultaneously. However, it has limitations, including variability in results due to factors such as agar depth, inoculum density, and incubation conditions. Additionally, it is not suitable for all types of bacteria or antibiotics, particularly those that do not diffuse well in agar or require specific growth conditions.

Related Tests

Other methods for determining antibiotic susceptibility include the broth microdilution method, which involves growing bacteria in broth media with different concentrations of antibiotics to determine the minimum inhibitory concentration (MIC), and the Etest, a gradient diffusion method that combines aspects of both the disk diffusion test and the determination of MIC.

See Also

• Antibiotic resistance
• Microbial culture
• Minimum inhibitory concentration