Molecular Diagnosis of Methicillin-Resistant Staphylococcus aureus Colonization

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Patrick R. Murray J. Clin. Microbiol. 2013, 51(12):4284. DOI: 10.1128/JCM.01659-13.

LETTER TO THE EDITOR

Molecular Diagnosis of Methicillin-Resistant Staphylococcus aureus Colonization Patrick R. Murray Becton Dickinson Diagnostic Systems, Sparks, Maryland, USA

rouillet-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 the first-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 frequency of drop-out mutants is variable, reported from 4.5% (2) to as 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-Assant et al. to report the frequency of drop-out mutants in their patient populations, as well as the frequency of false-positive molecular assays caused by the presence of MRCoNS.

REFERENCES 1. Trouillet-Assant S, Rasigade J-P, Lustig S, Lhoste Y, Valour F, Guerin C, Aubrun F, Tigaud S, Laurent F. 2013. Ward-specific rates of nasal cocolonization with methicillin-susceptible and -resistant Staphylococcus spp. and potential impact on molecular methicillin-resistant Staphylococcus aureus screening tests. J. Clin. Microbiol. 51:2418 –2420. 2. Huletsky A, Giroux R, Rossbach V, Gagnon M, Vaillancourt M, Bernier M, Gagnon F, Truchon K, Bastien M, Picard F, van Belkum A, Ouellette M, Roy P, Bergeron M. 2004. New real-time PCR assay for rapid detection of methicillin-resistant Staphylococcus aureus directly from specimens containing a mixture of staphylococci. J. Clin. Microbiol. 42:1875–1884. 3. Blanc D, Basset P, Nahimana-Tessemo I, Jaton K, Greub G, Zanetti G. 2011. High proportion of wrongly identified methicillin-resistant Staphylococcus aureus carriers by use of a rapid commercial PCR assay due to presence of staphylococcal cassette chromosome element lacking the mecA gene. J. Clin. Microbiol. 49:722–724.

Address correspondence to [email protected]. Ed. Note: The authors of the published article declined to respond.

ACKNOWLEDGMENT

Copyright © 2013, American Society for Microbiology. All Rights Reserved.

P.R.M. is WW Director, Scientific Affairs, for Becton Dickinson Diagnostic Systems.

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