MRSA with a Novel mecA Homologue

Mary E. Wilson, MD


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In This Article

Abstract and Introduction


A newly identified strain of methicillin-resistant Staphylococcus aureus evades detection by the genetic assays commonly used to confirm such resistance.


First reported in England in 1961, methicillin-resistant Staphylococcus aureus (MRSA) has spread globally in hospitals and the community. Methicillin resistance can spread when bacteria acquire a staphylococcal cassette chromosome mec (SCCmec) element with the mecA gene, which encodes a penicillin-binding protein (PBP) with low affinity for β-lactam antibiotics.

Phenotypic resistance is detected by measuring zones of bacterial growth inhibition around antibiotic-impregnated discs. Laboratories confirm resistance using genetic assays, such as mecA polymerase chain reaction (PCR), a DNA microarray, and a PBP latex agglutination assay. In two new studies, researchers describe S. aureus isolates that were phenotypically resistant to methicillin but tested negative for the mecA gene.

After identifying a novel MRSA-associated mecA homologue in a bovine isolate from England, García-Álvarez and colleagues tested existing human and bovine isolate collections for it. They found MRSA in 24 of 940 mecA-negative staphylococcal isolates from dairy cows with mastitis in England and Wales; the novel mecA homologue was present in 13 of the 24 isolates. The homologue was also detected in 51 of 74 mecA-negative human MRSA isolates: 12 of 16 from Scotland, 15 of 26 from England, and 24 of 32 from Denmark. In Denmark, where all MRSA isolates from the country are archived, the detection rate of the novel mecA homologue increased each year for isolates collected after 2007. In England, geographic association was seen among both human and bovine MRSA strains. Although most MRSA isolates with the novel homologue were collected after 2007, one human isolate from Denmark dated back to 1975, indicating that the homologue has been in S. aureus for at least 36 years.

Shore and colleagues identified two human MRSA isolates that lacked mecA by conventional PCR and DNA microarray screening. The SCCmec element was similar to that described by García-Álvarez and colleagues. The two patients, who were in hospitals in Ireland 160 km apart, had no known connection. The isolates were from the same lineage — clonal complex 130, which is found predominantly in cattle. The authors noted that the SCCmec mobile genetic element is relatively small, making it easier to spread.


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