Evaluation of a new selective medium for methicillin-resistant Staphyloccocus aureus

The aims of this study were to evaluate the performance of a new medium, desferrioxamine oxacillin tellurite egg-yolk mannitol salt agar (DOTEMSA) in detecting methicillin-resistant Staphylococcus aureus (MRSA) and then to compare this medium against the Public Health Laboratory Service (PHLS) recommendation of mannitol salt agar (Oxoid) with oxacillin (OMSA) and Baird-Parker medium with ciprofloxacin (BPC) for the isolation of MRSA. The individual selective agents contained in DOTEMSA were tested against isolates of coagulase-negative staphylococci (CNS) and the medium with all constituents was challenged with various bacteria. Routine screening specimens were plated out on OMSA, BPC and DOTEMSA and the plates were incubated and examined at 24 and 48 h. Tellurite, desferrioxamine and oxacillin each inhibited the majority of CNS isolates; only three (of 103) grew in the presence of all three agents. Sixty-two of 63 isolates of MRSA grew on DOTEMSA and 59 produced lipase. Most other bacteria were inhibited. In all, 184 MRSA isolates were isolated from 540 screening specimens. The sensitivity of OMSA, BPC and DOTEMSA was 42%, 81% and 51% at 24 h, and 60%, 89% and 89% at 48 h. At 48 h, the combination of BPC and DOTEMSA detected 99% of MRSA isolates. Seventy, 49 and one non-MRSA isolates needed investigation for each of the three media respectively. A proposed strategy for MRSA screening would use BPC and DOTEMSA, examining BPC at 24 h and both media at 48 h. Provisional reports could then be issued at 24 h on the basis of rapid agglutination tests to confirm isolates as S. aureus from BPC and at 48 h on the basis of typical colonies from DOTEMSA.