Intact cell mass spectrometry (ICMS) used to type methicillin-resistant Staphylococcus aureus: media effects and inter-laboratory reproducibility.

Intact cell mass spectrometry (ICMS) rapidly analyses the surface composition of microorganisms providing rapid, discriminatory fingerprints for identification and subtyping of important nosocomial pathogens such as methicillin resistant Staphylocccus aureus (MRSA). In this study, ICMS using matrix-assisted laser desorption ionisation time-of-flight mass spectrometry (MALDI TOF/MS) was assessed for the identification and subtyping of MRSA. An intra- and inter-laboratory reproducibility study was carried out and the effects of culture media (an important source of variation for ICMS) were also studied. Several media used for the cultural identification of MRSA were examined using a panel of well-characterised staphylococcal isolates (n=26). Six MRSA isolates were analysed over a 1-month period for intra-laboratory reproducibility on the same instrument and three different culture media. Spectra were consistent for each isolate between the four experiments on the same culture medium. Individual isolates produced different spectral profiles on different culture media. Spectra from organisms grown on Columbia blood agar contained more peaks (approximately 120) compared to Columbia agar (approximately 50) and methicillin mannitol salt agar (approximately 25). All 26 staphylococcal isolates were subjected to an inter-laboratory study on two MALDI instruments. For each isolate, the overall spectral profile was the same for each of the two instruments but the baseline threshold values was adjusted due to instrument differences in detector sensitivities. Differences between certain regions of the spectra reproducibly identified isolates belonging to the two major MRSA strains (EMRSA phage group 15 and 16). These results demonstrate ICMS with appropriate media selection is a rapid and reproducible technique for identification and discrimination of MRSA.