Dear Editor

I read with interest in the molecular testing approach described by J.E. Moore et al.[1] This testing approach utilizes the Oxoid salt meat broth (without selective reagents or antimicrobials) to enhance bacterial growth from the screening swabs. The molecular assay was based on the facts that the mecA gene locus is present in both MRSE and methicillin-resistant Staphylococcus aureus (MRSA) and that the femB gene is present only in Staphylococcus aureus (regardless of susceptibility to methicillin).

In their study of screening swabs obtained from the 75 patients, 2 patients were found to be have MRSA by the PCR assay (positive for both mecA and femB; lane 6 in Figure 1). However, a swab containing both MRSE and methicillin-susceptible S. aureus (MSSE), present simultaneously in the same patient, could also yield the same positive results for both genetic loci. How would this rapid molecular assay be able to differentiate between a patient colonized with MRSA and a patient colonized with both MSSA and MRSE (a more common clinical finding)?

Reference

J E Moore, B C Millar, M Crowe, J Buchanan, M Watabe, P G Murphy, X Yongmin, K Milligan, and A McClelland. Molecular determination of carriage of the mecA locus in coagulase negative staphylococci in screening swabs from patients in an intensive care unit. Mol Pathol 2003;56:63.