Susceptibility Methods

Standardized antimicrobial susceptibility testing (AST) methods and interpretations have been developed by several different standardization organizations. In the United States, Clinical Laboratory and Standards Institute (CLSI) guidelines and recommendations are the most commonly used. CLSI has published over 200 standards for laboratory testing, including AST testing using various methodologies. By standardizing both how susceptibility testing is performed as well as the interpretive guidelines, consistency can be better obtained from test to test and isolate to isolate.

To Review: The three common methods for AST testing are the Kirby-Bauer method, the E-test gradient strip method, and microbroth dilution panels.

The Kirby-Bauer method is also known as the disk-diffusion method. This method commonly uses Mueller-Hinton agar or Mueller-Hinton agar with blood where a standardized inoculum of a pure isolate is spread out on the agar plate, allowing time for the media to absorb the liquid inoculum, then the desired antimicrobials are placed firmly on the plate. Multiple disks can be placed on a plate as long as the disks are placed far enough apart to not cause overlap where the zone sizes cannot be measured. The plates are then incubated for a standardized time and zone sizes are measured and interpretive standards such as "Susceptible", "Intermediate", "Resistant", or "Non-Susceptible" are typically used for each antimicrobial tested.