Antimicrobial susceptibility testing

Antimicrobial susceptibility testing (AST) is done to detect antimicrobial resistance, and to determine the appropriate antimicrobial to use for treatment.

There are many methods of AST, but the simplest and most cost effective is the antimicrobial disc diffusion method. This method has been used since the 1950s and is still the most common method used today. The protocol will be demonstrated in the next session, and involves applying discs impregnated with a specific concentration of antimicrobial to an agar plate that has been inoculated with the bacterial isolate being tested. Once the disc is applied to the plate, the antimicrobial immediately diffuses into the surrounding agar.

If the bacteria is resistant to the antimicrobial, it will be able to grow around the disc. If susceptible, the bacteria will be unable to grow near the disc where the antimicrobial is present. The area around the disc where there is no growth is called the zone of inhibition. The diameter of the zone of inhibition is compared to a cut-off point where the bacteria is considered resistant (if the zone is smaller than the cut-off point) or susceptible (the zone is larger than the cut-off point). These cut-off points (also known as clinical breakpoints) are published by various regulatory bodies, and there are several breakpoint tables available for guidance. In the demonstration we will be using the European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoint tables as these are free to use and available to everybody.

The EUCAST tables also provide the recommended media and growth conditions for different bacteria, along with the recommended control organisms. EUCAST uses many sources to determine breakpoints, including dosing, pharmacokinetics, resistance mechanisms, minimum inhibitory concentration (MIC) distributions, zone diameter distributions, pharmacodynamics and epidemiological cut-off values (ECOFFs).

If you would like to learn more about clinical breakpoints, visit the EUCAST website: http://www.eucast.org/clinical_breakpoints/

The choice of antimicrobials you choose to test will depend on the reason for testing. If testing for clinical purposes, the choice of antimicrobials will depend on local antimicrobial stewardship policies, availability and the clinical situation of the patient (for example a skin infection and a blood infection may be treated differently). If testing for research reasons, you may be interested in testing a wider range of antimicrobials, depending on your research question.