Rapid direct lipid profiling of bacteria using desorption electrospray ionization mass spectrometry

Abstract

Desorption electrospray ionization (DESI) was employed to measure lipids directly from sixteen bacterial samples without extraction or other sample preparation. Differentiation of different bacterial species and some sub-species was achieved using either the positive or the negative ion mode DESI mass spectra covering the mass/charge range up to m/ z 1000. The data were confirmed by electrospray mass spectrometry (ESI-MS) of lipid extracts from the same bacterial samples. Although the signals were lower, the quality of the direct ionization DESI spectra compared favorably with that of the ESI spectra extracts prepared using chloroform/methanol. The use of unit mass resolution in these experiments allows for overlaps of nominally isobaric and isomeric lipids at particular m/ z values. Tandem mass spectrometry was performed to validate the presence of particular lipids falling into several classes of phospholipids, including phosphatidylethanolamines (PE), phosphatidylglycerols (PG) and lysophospholipids. In addition, lysyl-phosphatidylglycerol (LPG) and lipopeptides were observed in the cases of gram-positive bacteria. DESI-MS lipid profiling was applied to the characterization of four different bacterial species including thirteen Salmonella strains. Two bacterial samples Escherichia coli K-12 and Salmonella typhimurium INSP24 were also grown in three different media. Data were compared using principal component analysis (PCA), which indicated that the different species are readily distinguished and that different growth media do not prevent bacterial species differentiation in the cases examined. Several different Salmonella strains are also distinguishable from each other based on the PCA results.