Have you ever wondered how antimicrobial susceptibility testing works? Erdal Erol, DVM, MSc, PhD, of the Gluck Equine Research Center in Lexington, Ky., explains the process.

One of the principal functions of a microbiology laboratory, in order to best treat infections, is determining antimicrobial susceptibilities of bacterial isolates. This testing is performed on bacterial (and some fungal) isolates from clinical specimens if the isolate is a probable cause of an animal’s infection and the susceptibility of the isolate is uncertain. From the veterinarian’s point of view, the results of susceptibility tests are often considered as important–or more important–than identifying the pathogen involved. Therefore, the antimicrobial susceptibility test is routinely performed in microbiology laboratories.

Antimicrobial susceptibility testing predicts the effectiveness of treatment with antimicrobial agents tested against the isolated microorganisms. The Clinical and Laboratory Standards Institute (CLSI), an international, interdisciplinary, standards-developing, and educational organization established Interpretative criteria that determine whether the bacterial isolate is susceptible, intermediate, or resistant to the antimicrobials