Q: Please review interpretation of bacterial culture and sensitivity testing.

A: Dr. Dawn M. Boothe at the 2005 American College of Veterinary Internal Medicine (ACVIM) Forum in Baltimore gave a lecture on appropriate use of antimicrobial drugs. Some relevant points in this lecture are provided below.

Important considerations for selection of an antimicrobial are, in order of priority: 1) confirming the need; 2) identifying target organisms; and 3) identifying the target site.

Antimicrobial selection should strive for a spectrum that is as narrow as possible, thus avoiding the sequelae of drug use on the normal flora and unnecessary selection pressure. For complicated infections, identification should be based on culture and sensitivity; this information provides a basis for designing a dosing regimen most appropriate for the animal.

Basing antimicrobial selection on culture and sensitivity information does not guarantee success, just as failing to use culture and sensitivity as a basis for selection (or selecting a drug characterized by "R" on the information) does not guarantee failure. The "90-60 rule" implies that approximately 90 percent of infections treated based on culture and sensitivity are likely to respond if an "S" drug is selected; yet, up to 60 percent will respond even if an "R" drug is selected. The most likely situation where the latter is true is if the infection is at a site in which the drug is prescribed at a much higher dosage than that achieved in the test tube (i.e., much higher than the minimum inhibitory concentration (MIC).

Although culture and sensitivity information can be a powerful tool to guide selection, it nonetheless is an in vitro test applied to in vivo conditions; over-reliance on the information can contribute to therapeutic failure. Anaerobic infections are particularly problematic. Obligate anaerobes are exquisitely sensitive to increased oxygen tension and will not survive if exposed to oxygen. No growth may be mistakenly interpreted as lack of anaerobic infection. Many organisms are facultative anaerobes, capable of growth in anaerobic environments. Aerobic cultures may yield their growth, but the anaerobic environment in the animal may limit response to antimicrobials (particularly aminoglycosides). Organisms without cell walls (Mycoplasma, L-forms, etc.) and others difficult to culture, or slow-growing organisms (anaerobes, selected Gram positives, Nocardia, atypical Mycobacteria and others) and culture and sensitivity information may not include MIC.

Just as absence of growth does not indicate absence of infection, isolation of an organism is not necessarily evidence of infection, nor even if infection is present, does the isolated organism represent the infecting organism. Clearly, culture of an organism from a tissue that is normally sterile indicates infection. However, discriminating between normal and infecting flora can be difficult. Pure culture and large number of organisms are indicators of infection.

The culture and sensitivity procedures themselves are fraught with potential errors. For practices that provide in-house susceptibility testing, care should be taken to follow guidelines established and published by (or comparable to) the Clinical and Laboratory Standards Institute (CLASI) or comparable federal agency. Materials, including interpretive standards, should be validated by the appropriate agency. Minor changes in pH, temperature, humidity, etc. can affect results profoundly. Personnel should be trained specifically in culture techniques, and hospitals that provide this service should maintain well-designed and adequately collected quality control data to validate their procedures.