Characterisation of Staphylococcus aureus lipids by nanoelectrospray ionisation tandem mass spectrometry (nESI-MS/MS)

Staphylococcus aureus is a major opportunistic pathogen that is exposed to antimicrobial innate immune effectors and antibiotics that can disrupt its cell membrane. An understanding of S. aureus lipid composition and its role in defending the cell against membrane-disrupting agents is of fundamental importance. Common methods for characterising lipid profiles suffer shortcomings such as low sensitivity of detection and inferior resolution of the positional assignments of fatty acid chains in lipids. This present study developed a rapid and sensitive nano-electrospray ionisation tandem mass spectrometry (nESI-MS/MS) method to characterise the lipid composition of three commonly studied S. aureus isolates: Newman, Mu50 and BB270. Confirming previous studies, nESI-MS/MS revealed that phosphatidylglycerols were most abundant in S. aureus membranes, while diglucosyldiacylglycerols and lysyl-phosphatidylglycerols were also detected. Positional assignments for individual fatty acid chains within these lipids were also determined. Concomitantly, gas chromatography mass spectrometry of the fatty acids validated the molecular characterization and showed the principal species present in each strain were predominately anteiso- and iso-branched chain fatty acids. Though the fatty acid and lipid profiles were similar between the S. aureus strains, this method was sufficiently sensitive to distinguish minor differences in lipid composition. In conclusion, this nESI-MS/MS methodology can characterise the role of lipids in antimicrobial resistance, and may even be applied to the rapid diagnosis of drug-resistant strains in the clinic.