Detection of methicillin-resistant coagulase-negative staphylococci harboring the class A mec complex by MALDI-TOF mass spectrometry.

The aim of this study was to test the identification of methicillin resistance in coagulase-negative staphylococci by routine matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS). SCCmec cassettes of type II, III and VIII encode a small peptide called PSM-mec in the vicinity of mecA. It is visible at m/z 2415 during MALDI-TOF MS of whole cells of Staphylococcus aureus. In view of the fact that psm-mec has been identified in methicillin-resistant coagulase-negative staphylococci, we evaluated a collection of clinical coagulase-negative staphylococci, that contained 77.03% of methicillin-resistant isolates, for the presence of the structural gene encoding PSM-mec and the appearance of the corresponding signal during mass spectroscopy. In MALDI-TOF MS spectra, 89.65% of the strains that harbored the gene yielded the correct signal, corresponding to a sensitivity of 0.897 and a specificity of 1.0. However, regarding detection of methicillin resistance, i. e. considering all resistant strains as positive regardless of the presence of the gene, the overall sensitivity of the test decreased to 0.285, due to the fact that only 29.43% of all resistant isolates contained psm-mec. In conclusion, the presence of the signal in MALDI-TOF MS quickly indicates methicillin-resistance in coagulase-negative staphylococci but its absence does not indicate susceptibility to methicillin.