Evaluation of commercial chromogenic media for the detection of meticillin-resistant Staphylococcus aureus

Summary Background Selective chromogenic media allowing one-step meticillin-resistant Staphylococcus aureus (MRSA) isolation and identification are widely used. However, the changing epidemiology of MRSA means that the suitability of these chromogenic media requires investigation. Aim To evaluate the following chromogenic media – Colorex MRSA, MRSA Select II, ChromID MRSA, and MRSA Brilliance 2 – for the detection of divergent strain types. Methods We used a diverse collection of S. aureus , including strains harbouring the mecC gene, strains expressing varying levels of meticillin resistance, and isolates recovered from patient samples. Findings MRSA Select II, Colorex MRSA, and ChromID each grew at a density of 1.5 × 10 1 cfu/mL for each SCC mec type investigated. Brilliance 2 demonstrated growth at 1.5 × 10 1 cfu/mL for mecC MRSA but at a higher density (1.5 × 10 4 cfu/mL) for the three mecA MRSA strains. All four media demonstrated excellent sensitivity for MRSA detection (≥99%), but reduced levels of specificity (85–73%) when challenged with a range of meticillin-susceptible S. aureus (MSSA) isolates. High levels of false positives (∼50%) were also obtained with all chromogenic media when tested with mec -negative borderline oxacillin-resistant S. aureus (BORSA) isolates. Conclusion Although false positives may be obtained with some strains of MSSA and BORSA, the high sensitivity of these media and their ability to recover almost all MRSA tested (including oxacillin-susceptible and mec C-positive strains) confirm the value of chromogenic agar in MRSA detection.