Molecular Diagnosis of Methicillin-Resistant Staphylococcus aureus Colonization

Abstract

Trouillet-Assant et al. (1) described the difficulties in developing a molecular methicillin-resistant Staphylococcus aureus (MRSA) screening assay. Cocolonization with methicillin-susceptible S. aureus (MSSA) and methicillin-resistant coagulase-negative staphylococcus (MRCoNS) led to false-positive results in thefirst-generation assays (multiplex assays targeting the mecA gene and a S. aureusspecific gene). Likewise, drop-out mutants contributed to falsepositive results in the second-generation assays (multiplex assays that target orfX-staphylococcal cassette chromosome mec element [SCCmec] junction sequences). For this reason, third-generation assays were developed (multiplex assays that target mecA/mecC genes, orfX-SCCmec junction sequences, and a S. aureus-specific gene). A false-positive reaction can still occur in the presence of a MRCoNS strain but only if the MSSA strain is a drop-out mutant. The frequencyofdrop-outmutantsisvariable,reportedfrom4.5%(2)toas high as 12.5% (3). Recognizing that the prevalence of drop-out mutants can be influenced by clonal spread, it would still be useful for Trouillet-Assantetal.toreportthefrequencyofdrop-outmutantsin their patient populations, as well as the frequency of false-positive molecular assays caused by the presence of MRCoNS.