Cepheid was one of the first companies to market a methicillin-resistant Staphylococcus aureus (MRSA) assay from nares swab specimens, based on its SmartCycler®real-time PCR platform.
Molecular detection of methicillin resistance in staphylococci is based on detecting the mecA gene. However, since coagulase-negative staphylococci (CNS) can also possess this gene, discrimination between CNS and MRSA must be achieved by detecting additional gene sequences specific to S. aureus. Cepheid's multiplex assay included targets for six variants of the mecA gene and the S. aureus orfX gene. Despite this, independent investigators documented incidences of both false positives and false negatives.
Since this time, Xpert MRSA/SA BC (blood culture) test cartridges have been developed for use with blood from positive blood culture bottles. The test has three targets: the gene encoding staphylococcal protein A (spa), the methicillin resistance gene mecA (mec), and the junction region between orfX in the S. aureus chromosome and the SCCmec element. The MRSA/SA SSTI was developed for skin and soft tissue samples.
The BD GeneOhm™ MRSA assay is another real-time assay designed for the SmartCycler® platform. This assay employs molecular beacons for detection. The probe has a hairpin shape, with a fluorophore at one end and a quencher at the other. Without the target, the hairpin is closed, and fluorescence is quenched. In the presence of the target, the hairpin opens when the beacon hybridizes with the target, resulting in fluorescence emission measured during each amplification cycle. The availability of results is similar to that of the Cepheid assay. As with the Cepheid assay, independent investigators documented some incidence of both false positives and false negatives but noted the advantage of the rapid availability of screening results for surveillance purposes.
Roche Diagnostics has also offered several MRSA assays, beginning with the LightCycler® 2.0 instrumentation, then available on the high-throughput Cobas® 4800 system, and now on the Liat® point-of-care instrument. The progression of the real-time PCR method through the years to that of MRSA/SA targets and the different platforms demonstrates how important it is for a laboratory to choose a platform size that fits the needs of its population.
Note: It is important to remember that these tests depend upon the specimen type validated by the manufacturer. Always research and read vendor methodology and package inserts carefully. All methods have documented false positives and negatives, and product limitations are listed in manufacturer information.
