Clinical microbiology and infection vol:vol. 11 issue:s2 pages:391-545
European Congress of Clinical Microbiology and Infectious Diseases edition:15th location:Copenhagen date:April 2-5th 2005
Introduction: Numerous primary media have been advocated for the enhanced detection of methicillin-resistant Staphylococcus aureus (MRSA). Recently, a lipovitellin-salt-mannitol-agar (LSM) with an oxacillin disk has been described. On this medium, detection of MRSA isolates is enhanced due to production of a lecithinase, which results in enlarged colonies surrounded by a zone of opacification. Also, the use of a cefoxitin disk method has recently been described. It has been reported as being superior to the oxacillin disk method.
Aim: To evaluate the performance of LSM with a 1 lg oxacillin disk and a 30 lg cefoxitin disk (LSMoc) as selective and differential primary medium for detection of MRSA from
Materials and methods: Screening specimens (swabs of nose and perineum) were obtained from a patient population at the intensive care unit. The LSMoc medium was compared with an in house method using colistin-nalidixic acid agar (CNA) with 5% horse blood by performing parallel inoculation. S. aureus isolates were presumptively identified by colonial morphology and confirmed by coagulase tube test and Vitek 2 ID-GPC testcard. Methicillin resistance on the LSMoc medium was presumptively detected by reading the zones of inhibition around both disks and confirmed by disk diffusion and PBP2’ latex agglutination assay.
Results: 149 (19.7%) of 754 screening specimens were positive for MRSA by one or both methods. Sensitivity of the LSMoc medium (90%) was higher than sensitivity of the in house method (76%) (P < 0.01). However, in contrast to the CAN medium, the LSMoc medium required an additional 24–48 hours of incubation before it could be examined for the presence of MRSA. The LSMoc medium misidentified 22 isolates as MRSA (specificity 96.4%). These were mostly coagulasenegative Staphylococci, mainly belonging to the species S. capitis, which gave opacification. There was no difference in the number of MRSA isolates detected on the LSMoc medium by use of the oxacillin or cefoxitin disk.
Conclusion: Use of the LSMoc medium increases the yield of MRSA, due to added enrichment for growth and ease of visual recognition of MRSA isolates. However, the medium still requires confirmation of presumptive MRSA isolates and requires prolonged incubation.