Identification of agr-positive methicillin-resistant Staphylococcus aureus harbouring the class A mec complex by MALDI-TOF mass spectrometry.

A small peptide called PSM-mec is encoded on the type II, III and VIII SCCmec cassettes present in the genomes of nosocomial methicillin-resistant Staphylococcus aureus (MRSA) strains. This peptide is excreted by agr-positive strains, which represent about 89% of the strains of our collection and can be identified by the presence of delta toxin in mass spectrometry. The presence of the peptide in the MALDI-TOF MS spectra of whole cells was proved by a knock-down experiment employing a clone that expressed antisense RNA to psm-mec. Furthermore, evaluation of a collection of clinical agr-positive MRSA and MSSA isolates and type strains showed that, using a detection window of m/z 2411-2419, the PSM-mec is detected by mass spectrometry of whole cells with a sensitivity of 0.95 and a specificity of 1, thereby enabling rapid identification of a subgroup of MRSA with a method that is used during routine identification procedures.