The presence of a single MALDI-TOF mass spectral peak predicts methicillin resistance in staphylococci

- Routine clinical MALDI-TOF MS can detect PSM-mec at 2415 m/z in staphylococci.
- PSM-mec detection has a high positive predictive value for inferring mecA carriage.
- Most S. aureus and S. epidermidis that carry mecA lack detectable levels of PSM-mec.

The class A mec gene complex also carries the psm-mec gene. The psm-mec gene product can be detected as a peak near 2415 m/z using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). The correlation of a 2415 m/z peak with methicillin resistance (mecA carriage) in consecutive staphylococcal blood culture isolates was evaluated. A 2415 ± 2.00 m/z peak was observed in 37% (51/137) of methicillin-resistant Staphylococcus aureus, in none (0/146) of the methicillin-susceptible S. aureus, in 6% (10/180) of methicillin-resistant S. epidermidis, and in 2% (1/63) of methicillin-susceptible S. epidermidis isolates. Although the sensitivity of the 2415 m/z peak for mecA carriage in S. aureus and S. epidermidis was low (37% and 6%, respectively), the specificity was high (≥98%). This peak can be identified during routine MALDI-TOF MS clinical testing, and the analysis adds no reagent cost and requires minimal time expenditure.