Antibiotic sensitivity testing

Antibiotic sensitivity test

Antibiotic sensitivity testing is a laboratory method used in microbiology to determine the susceptibility of bacteria and fungi to antibiotics. This test is crucial in clinical settings to help guide physicians in their choice of treatment for infectious diseases.

Overview

Antibiotic sensitivity testing, also known as antibiotic susceptibility testing (AST), is performed to identify which antibiotic will be most effective in treating a bacterial infection in vivo. The results of the test are commonly reported as 'sensitive', 'resistant' or 'intermediate'.

Methodology

There are several methods for antibiotic sensitivity testing. The most commonly used methods are the disk diffusion test (Kirby-Bauer method), the Etest (or epsilometer test), and the broth dilution method.

Disk Diffusion Test

In the disk diffusion test, a bacterial suspension is spread over the surface of a agar plate. Antibiotic-impregnated disks are then placed on the agar surface. After incubation, the diameter of the zone of inhibition around each disk is measured and compared with standard tables to interpret whether the organism is sensitive, resistant, or intermediate to the antibiotic.

Etest

The Etest is a quantitative method that provides an antibiotic concentration gradient for the determination of the minimum inhibitory concentration (MIC).

Broth Dilution Method

In the broth dilution method, the MIC is determined by inoculating the organism into a series of tubes or wells containing a range of antibiotic concentrations.

Interpretation

The results of antibiotic sensitivity testing are interpreted using breakpoints. Breakpoints are the antibiotic concentrations at which bacteria are classified as sensitive, intermediate, or resistant.

Clinical Significance

Antibiotic sensitivity testing is crucial in guiding antibiotic therapy, especially in the era of increasing antibiotic resistance. It helps in selecting the most effective antibiotic to treat an infection and in preventing the overuse of broad-spectrum antibiotics.

See Also

• Antimicrobial resistance
• Pharmacodynamics
• Pharmacokinetics