Antibiotic susceptibility

Antibiotic Susceptibility is the measure of the sensitivity of bacteria to antibiotics. It is used in microbiology laboratories to guide the choice of antibiotics to be used in treating bacterial infections.

Overview

Antibiotic susceptibility testing (AST) is a laboratory method used to determine how an organism, such as bacteria or yeast, responds to being exposed to an antibiotic. This is important in clinical settings to help guide physicians in treatment decisions.

Methodology

There are several methods for determining antibiotic susceptibility, including disk diffusion, broth dilution, and automated systems. The choice of method depends on the type of organism being tested, the potential treatment options, and the laboratory's resources and capabilities.

Disk Diffusion

In the disk diffusion method, a bacterial sample is spread over the surface of a agar plate. Disks impregnated with specific antibiotics are then placed on the plate. The plate is incubated to allow bacterial growth. If the bacteria are susceptible to the antibiotic, a clear zone, or "zone of inhibition," appears around the disk where bacterial growth has been prevented.

Broth Dilution

In the broth dilution method, the bacteria are grown in a liquid medium. Different concentrations of the antibiotic are added to the broth. The lowest concentration that prevents visible growth of the bacteria is known as the minimum inhibitory concentration (MIC).

Automated Systems

Automated systems use machines to perform susceptibility testing. These systems can test multiple antibiotics and organisms at the same time, and often include software that can interpret the results.

Interpretation of Results

The results of antibiotic susceptibility testing are usually reported as one of three categories: susceptible, intermediate, or resistant.

• Susceptible means that the organism is likely to be inhibited by the usual concentrations of the antibiotic.
• Intermediate means that the organism may be inhibited by high concentrations of the antibiotic, and/or that the clinical effectiveness of the antibiotic is uncertain.
• Resistant means that the organism is not inhibited by the usual concentrations of the antibiotic.

Clinical Significance

Antibiotic susceptibility testing is crucial in guiding antibiotic therapy, especially in the era of increasing antibiotic resistance. It helps to ensure that patients receive the most effective treatment for their infections, and can also help to prevent the overuse of broad-spectrum antibiotics, which can contribute to the development of resistance.

Antibiotic susceptibility Resources
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