Importance of Antimicrobial Susceptibility Testing

Conducting a culture and susceptibility is the best way to determine the bacterial pathogens associated with disease in an animal and to guide selection of the appropriate antimicrobial. In many cases the clinician will need to select an antimicrobial for use prior to the availability of culture and susceptibility data. To do this, the clinician selects an antimicrobial that is likely to be effective against the organisms most commonly identified as pathogens of the body site of interest. Clinicians should resist the urge to simply choose the antimicrobial with the broadest spectrum, as using antimicrobials in this way may lead to increased resistance in the population. It has been estimated that approximately 40 percent of the time, antibiotic therapy should be changed after receipt of susceptibility testing results.

The cardinal rule of appropriate antimicrobial use is to use the right drug, dose, and dosing interval to achieve a sufficient concentration of the antimicrobial at the site of infection. Failure to adhere to this principle promotes antimicrobial resistance and therapeutic failure.

Antimicrobial Susceptibility and Isolates

We perform antimicrobial susceptibility testing on isolates that, after consideration of the body site and disease condition, may reasonably be expected to be involved in the disease process. Testing of isolates that are common contaminants or normal flora would not provide useful information and might lead to inappropriate antibiotic use. You are always welcome to contact the lab and request additional testing in special circumstances. There may be an additional charge for further testing. Another reason that a susceptibility test may not be done is that either the organism does not grow sufficiently well to meet the requirements of the tests or that our lab does not currently conduct testing on the particular type of organism isolated (anaerobes, mycobacteria, fungi, and mycoplasma).

Interpreting Susceptibility Results

Fig. 1: Plates containing the dilutions of appropriate antimicrobials.

The Bacteriology Section tests the susceptibility of bacterial isolates to a wide variety of antimicrobial agents. We use panels of antibiotics (see Antimicrobial Susceptibility Testing Resources) which are tested for routinely based on the bacterial species, the animal source of the isolate, and, in some cases, the body site affected. Special panels of antibiotics are also available for topical therapy for eye infections and for mastitis pathogens. In addition to our routine panels, we can test for susceptibility to a number of other drugs upon special request or for highly resistant bacteria. The drugs we have chosen to test for are determined by label indications for the drugs, various federal guidelines about the use of certain antibiotics in certain species, and clinical standards.

There are two primary methods used to determine the susceptibility of an organism to various antibiotics. Our preferred, and most commonly used, method of determining antimicrobial susceptibility is to use the broth micro-dilution method (Fig. 1), which provides results in a quantitative manner as a Minimum Inhibitory Concentration (MIC). The MIC is then used to provide a qualitative result of Sensitive, Intermediate, or Resistant. So, results using this method are provided as the MIC and a qualitative result of Sensitive, Intermediate, or Resistant. Please see Antimicrobial Susceptibility Testing Resources for more information on how to use the susceptibility data provided by the MSU VDL.

Fig. 2: Zones of inhibition of bacterial growth around antimicrobial disks. (Blood-MH agar).

In some cases the broth micro-dilution method is not an option (fastidious bacterial isolate, topical antibiotics) and in that case, the disk diffusion method (also known as the Kirby Bauer method) is used. This method involves plating the bacteria of interest on a solid agar plate and dropping antibiotic-impregnated disks onto the surface of the plate and measuring the zones of clearing (inhibition of bacterial growth) around the disks (Fig. 2). The zone size is then used to provide a qualitative result of Sensitive, Intermediate, or Resistant. So, results using this method are provided as only the qualitative result of S, I, or R.