Mass spectrometric identification of phenol-soluble modulins in the ATCCA® 43300 standard strain of methicillin-resistant Staphylococcus aureus harboring two distinct phenotypes

Staphylococcus aureus subsp. aureus Rosenbach (ATCCA (R) 43300 (TM)) is a representative methicillin-resistant S. aureus (MRSA) strain that is used as a quality control strain for testing the methicillin susceptibility of clinical isolates. It has been consistently observed that colonies with two different colors (golden yellow and white) grow from the ATCC stock on blood agar plates. In this study, staphylococcal peptide and protein profiling was performed using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. From the extract of the potentially hemolytic 43300 strain, we identified a single significant peak at an observed mass-to-charge (m/z) value of 2306.9, which correlates well with the predicted mass of formylated phenol-soluble modulin alpha 2, a major staphylococcal virulence factor. Subsequent liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis revealed that the hemolytic 43300 cells predominantly produced various phenol-soluble modulins (PSMs) and their export proteins, including four alpha type PSM peptides, PSM beta 1, PSM-mec, PmtC and PmtD. However, none of the PSM peptides was detected in the presumably non-hemolytic 43300 strain, but the export proteins PmtC and PmtD. We found that this MRSA standard strain expresses two distinct phenotypes and that their phenotypic features are more likely associated with PSM toxin production. Therefore, careful attention is needed when MRSA reference strains are utilized for the diagnosis and susceptibility testing of MRSA in clinical settings.