How do you test for antibiotic resistance in bacteria?

Antibiotic resistance in bacteria is tested using methods like the disk diffusion test and the broth dilution test.

The disk diffusion test, also known as the Kirby-Bauer test, is a common method used to test antibiotic resistance. In this test, a bacterial sample is spread evenly across a nutrient-rich agar plate. Small disks impregnated with specific antibiotics are then placed on the agar. The plate is incubated to allow the bacteria to grow. If the bacteria are sensitive to a particular antibiotic, a clear zone, known as a zone of inhibition, will appear around the disk where the bacteria have not grown. The size of this zone is measured and compared to standard tables to determine whether the bacteria are resistant, intermediate, or sensitive to the antibiotic.

The broth dilution test is another method used to determine the minimum inhibitory concentration (MIC) of an antibiotic, which is the lowest concentration of an antibiotic that prevents visible growth of a bacterium. In this test, a series of test tubes containing equal volumes of a nutrient broth and increasing concentrations of an antibiotic are inoculated with a standard amount of bacteria. The tubes are then incubated. The MIC is the concentration of antibiotic in the first tube in the series that shows no visible growth after incubation.

Another method is the E-test, which uses a strip impregnated with a gradient of an antibiotic. The strip is placed on an agar plate inoculated with the bacteria, and after incubation, the MIC is read where the edge of the bacterial growth intersects the strip.

These tests are crucial in clinical settings to guide the choice of antibiotics for treating bacterial infections. They also play a vital role in monitoring the emergence and spread of antibiotic-resistant bacteria.