Antimicrobial Susceptibility Testing

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.

There are two main methods of determining antimicrobial susceptibility:

  • The Kirby-Bauer (or disc diffusion) method is inexpensive, flexible, and one of the more established methods used. This method involves swabbing a standardized suspension of a bacterial isolate over the surface of an agar plate and placing paper discs containing a concentration of a single antimicrobial drug on the inoculated surface. After overnight incubation, the diameters of the zones produced by drugs are measured, and per CLSI guidelines, the isolate is interpreted as susceptible, resistant, or intermediate. Unfortunately, the qualitative interpretive criteria for veterinary pathogens and drugs for the Kirby-Bauer method are limited. Additionally, the system is not automated, and antibiotic susceptibility tests cannot be performed on some slow-growing bacteria (such as nocardioform bacteria, Listeria, and some mycobacteria), anaerobic bacteria, and fungi.
  • The second most common method, the broth microdilution method, provides antimicrobial susceptibility results with minimum inhibitory concentration (MIC) levels. The MIC levels represent the lowest concentration of drug required to inhibit the growth of a bacterial isolate. Broth microdilution methods are used with both commercial automated and semi-automated instruments. Most current instruments work on the principle of turbidimetric detection of bacterial growth in a broth medium by use of a photometer to examine the test wells (the sample being clear or less turbid (cloudy) when the drug inhibits bacterial growth). The broth microdilution (or MIC) method is a more expensive method but includes a broader and updated number of antimicrobials for veterinary pathogens. The MIC method is also able to determine antimicrobial susceptibility patterns for nocardioform and other slow-growing bacteria, some fungi (yeasts), and some anaerobic bacteria that cannot be assessed by Kirby-Bauer disk diffusion.

The University of Kentucky Veterinary Diagnostic Laboratory and other diagnostic laboratories have started using the MIC method for the bacteria (and yeast) isolated in animal specimens, believing this new method will better serve clients. The Kirby-Bauer method will also be available upon request from practitioners.

CONTACT: Erdal Erol, DVM, MSc, PhD, 859/257-8283,, University of Kentucky Veterinary Diagnostic Laboratory, Lexington, Kentucky

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd's, London, brokers, and their Kentucky agents.

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Equine Disease Quarterly

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