Antimicrobial Susceptibility Testing (AST) – A procedure used to determin which antibiotics a specific organism or group of organisms are susceptible to.
Pathogens isolated from clinical specimens are identified to confirm medical disagnoses and to guide antimicrobial therapy to ensure the correct antibiotics are used and avoid using antibiotics which the pathogen may be resistant to.
The standard procedure for assessing antimicrobial activity is the disc diffusion test. Agar is evenly innoculated with a suspension of pure culture onto the surface. Filter paper discs containing a specified dose of antimicrobial agent are placed onto the innocluated 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 catagories 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 utilises an antibiotic dilution assay in agar, within culture tubes or within microtitre plates. When using the MIC method within culture tubes or microtitre plates, serial dilutions of antibiotics are innoculated into the well or tubes alongside a standard innoculum 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 innoculated diffusion strip containing a antibiotic concentration gradient is applied to the agar. When applied ot 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.