Limitations of a chromogenic agar plate for the identifying bacteria isolated from equine endometritis samples

Background: The use of commercial chromogenic agar plates for the rapid, easy and correct identification of equine endometritis-causing bacteria has been proposed. Preliminary tests in our lab revealed undescribed limitations. Therefore, we tested the ability of the Brilliance UTI agar, a commercially available chromogenic agar, to accurately identify bacteria causing equine endometritis. Objectives: To 1) investigate whether bacteria present in the equine uterus are able to grow on this chromogenic agar plate, 2) determine whether these bacteria belong to the genera for which these agar plates were originally designed and 3) consider whether these bacterial genera can be correctly identified, based only on the colour appearance. Study design: In vitro experiments. Methods: Macroscopic growth and colour appearance on the Brilliance UTI agar of 58 bacterial isolates, from previously collected equine uterine samples, were evaluated. Isolates were tentatively identified at the genus level using the manufacturer's guidelines and results were compared with MALDI-TOF MS as a "gold standard". Results: The study revealed that 1) 77% (N = 45/58) of the bacterial isolates grew well on this chromogenic agar, 2) 83% of the investigated isolates (N = 48/58) belonged to one of the genera for which guidelines for identification were provided by the manufacturer and 3) only 50% of the isolates (N = 29/58) were correctly identified at the genus level, based only on colour appearance. Main limitations: The current study uses purified bacterial isolates to inoculate the chromogenic agar plates, instead of fresh uterine samples. Bacteria were identified to the genus level using MALDI-TOF MS. Conclusion: This study shows that identification at the genus level based only on colour appearance, without additional tests or the simultaneous use of other media, is not reliable based on the existing identification guidelines