Antimicrobial Susceptibility Testing (AST) – A procedure used to determine which antibiotics a specific organism or group of organisms are susceptible to.

Pathogens isolated from clinical specimens are identified to confirm medical diagnoses and to guide antimicrobial therapy to ensure the correct antibiotics are used and avoid using antibiotics that the pathogen may be resistant to.

The standard procedure for assessing antimicrobial activity is the disc diffusion test. Agar is evenly inoculated with a suspension of pure culture onto the surface. Filter paper discs containing a specified dose of antimicrobial agent are placed onto the inoculated agar. After a specified incubation period, the diameters of the inhibition zones formed around each disc are measured. Inhibition zone diameters are then interpreted into susceptibility categories based on the zone size (Susceptible, Intermediate, Resistant) set by organisations such as EUCAST or CLSI.

Another procedure now being more commonly used is the MIC (minimum inhibitory concentration) method. The MIC procedure utilizes an antibiotic dilution assay in agar, within culture tubes or within micro titre plates. When using the MIC method within culture tubes or micro titre plates, serial dilutions of antibiotics are inoculated into the well or tubes alongside a standard inoculum of a test organism. Growth in the presence of each antibiotic concentration is measured using turbidity. Antibiotic susceptibility is stated as the highest dilution or lowest concentration of antibiotic that completely inhibits growth. When using the MIC method on agar, an inoculated diffusion strip containing an antibiotic concentration gradient is applied to the agar. When applied to the agar, the gradient transfers from the strip into the agar. After overnight incubation or longer, an elliptical zone of inhibition centered around the strip forms. The MIC value can be read at the point the ellipse edge intersects the MIC strip. The resulting MIC value can then be interpreted using standards such as EUCAST or CLSI.