Discrimination of aerobic endospore-forming bacteria via electrospray-ionization mass spectrometry of whole cell suspensions

Direct injection electrospray ionization mass spectrometry (ESI-MS) without prior analyte separation was investigated for the analysis of whole cell s uspensions of bacteria. Thirty-six strains of aerobic endospore-forming bac teria, consisting of six Bacillus species and one Brevibacillus species, we re studied. ESI was performed in the positive ion mode on the bacterial sus pensions. Several peaks in the range of 250-1500 m/z were observed to contr ibute to variations in the spectral information among the species. Applicat ion of cluster analysis to the spectral data showed that this ESI-MS techni que was capable of discriminating strains of the species B. subtilis. This investigation demonstrates the feasibility of measuring liquid samples with minimal sample preparation that can be useful for discrimination at the su bspecies level. A change in the cone potential in the electrospray ion sour ce was found to influence the spectral information of representative strain s of all of the seven species tested. This has implications,with respect to optimizing the experimental conditions for discriminatory purposes, but co uld offer additional information with respect to microbial characterization . A comparison of the spectra of whole cell suspensions and cell-free super natants showed a high degree of similarity between the two, which has conse quences for ease of automation.