Differentiating Antibiotic-Resistant Staphylococcus aureus Using Secondary Electrospray Ionization Tandem Mass Spectrometry

Secondary electrospray ionization mass spectrometry (SESI-MS) is an innovative metabolomics approach that primarily focuses on the gas-phase analyte detection. In this study, we developed a secondary electrospray ionization tandem mass spectrometry (SESI-MS/MS) method with a homemade SESI-MS front end, for sensitive, reproducible and selective detection of headspace volatile organic compounds (VOCs) emitted from bacterial culture. The optimized SESI-MS/MS was applied to examine the VOC metabolome of a pair of isogenic methicillin-susceptible and resistant Staphylococcus aureus (MSSA and MRSA) strains. From the headspace of bacterial culture, twelve organic acids, and eight amino acids with the mass range of 40–300 Da were specifically targeted with two selected reaction monitoring (SRM) transitions. Our results indicated that MSSA and MRSA strains can be clearly differentiated via partial least-squares discriminant analysis (PLS-DA) based on their headspace metabolic profiles. Furthermore, we studied the stress response of MSSA and MRSA to antibiotics treatment. Our result showed that MSSA and MRSA generated dramatically changed VOC metabolic profiles in response to ampicillin, which indicated that SESI-MS/MS could also be used for antibiotic treatment response monitoring in future studies. This study showed that SESI-MS/MS VOC analysis provides an additional approach to the bacterial metabolome detection complementary to traditional aqueous phase metabolite analysis. To the best of our knowledge, this was the first time that SESI-MS/MS was applied to investigate the bacterial metabolic perturbations caused by antibiotic treatment.