How to Diagnose Common Equine Reproductive Tract Bacterial Pathogens Using Chromogenic Agar

The most common microbiological pathogens of reproductive tract disease in the horse are Streptococcus sp., Escherichia coli, Klebsiella pneumoniae, Staphylococcus aureus, and Pseudomonas aeruginosa. The standard approach to the detection of these pathogenic bacteria is to culture the reproductive tracts of the mare or stallion. One or more general purpose media such as horse blood agar (HBA) or MacConkey agar are inoculated before aerobic incubation occurs at 37°C for 24–48 h. The identification of pathogenic bacteria has been based on colony size, morphology, pigmentation, hemolytic patterns, and gram-stain. Such colonial characteristics rarely permitted more than a presumptive identification, and biochemical and/or serological tests are required for definitive diagnosis. This approach frequently necessitates the testing of commensal bacteria that resemble pathogens, which may be more expensive and labor intensive, especially in a veterinary practice trying to make accurate and expedient bacterial diagnoses. The principle of the use of chromogenic agar is that chromogenic substrates are incorporated into the agar to reveal the genusor species-specific enzyme activities of microorganisms. These chromogenic substrates release specific colored dyes when hydrolyzed by enzymes of pathogenic microorganisms, which results in readily identifiable colored colonies. The inclusion of multiple chromogenic substrates into the culture media facilitates the differentiation of polymicrobial cultures by causing the development of multiple colored colonies. Selective chromogenic media were first used in 1976 for the direct identification of Escherichia coli in the primary culture of urine. Since then, a number of commercially prepared chromogenic agars have been developed for the accurate and rapid identification of pathogens of the human urinary tract, because urine cultures in human hospitals contribute greatly to the daily workload of a microbiological laboratory. Brilliance Urinary Tract Infection (UTI) Agar is one such commercially prepared media (formerly Chromogenic UTI agar). It allows the identification of Streptococcus sp., Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus sp., Enterobacter aerogenes, Enterococcus faecalis, Citrobacter sp., Proteus sp., and Candida sp.