Coagulase-negative staphylcococci lead to false-positive results on chromID S. aureus and chromID MRSA agar

1 ) Summary While Staphylococcus (S.) aureus is a frequent cause of infections and intoxications and causes severe losses for the dairy industry, coagulase-negative staphylococci (CNS) are commonly considered benign commensals. Correct identification of S. aureus and differentiation from CNS is crucial for diagnostic purposes. SAID and chromID MRSA agar plates (bioMerieux, FR) are selective chromogenic agar plates that were reported to allow for highly specific and sensitive identification of S. aureus and MRSA (methi- cillin-resistant S. aureus). However, when using these media in our laboratory, we noticed incongruent identification results, prompting us to question the specificity of SAID and chromID MRSA agar. In this study, we aimed to evaluate the performance of SAID and chromID MRSA agar by identifying organisms that may lead to false-positive results. To this end, we used the isolates that had yielded inconsistent identification results (n = 3), as well as 15 CNS strains representing S. chromogenes, S. cohnii, S. xylosus, S. sciuri, S. haemolyticus, S. epidermidis, S. simulans, S. warneri, S. equorum, S. hyicus, S. succinus, S. fleurettii, and S. lentus. Species identification was performed using matrix-assisted laser desorption and ionization time-of-flight mass spectrometry. The phenotype of all isolates on SAID agar and of cefoxitin-resistant S. fleurettii, S. sci- uri, S. succinus, and S. lentus isolates on chromID MRSA agar was evaluated. For SAID, we detected false-positive results for S. fleurettii, S. sciuri, S. succinus, S. lentus, S. vi- tulinus, Arthrobacter nicotianae, and Micrococcus luteus. For chromID MRSA, cefoxi- tin-resistant S. fleurettii, S. sciuri, S. succinus, and S. lentus led to false-positive results. In the light of our findings, screening for MRSA using chromID MRSA agar only may lead to false-positive results, potentially leading to severe therapeutic mistakes. We therefore strongly advise combination of the chromID MRSA agar with additional tests.