MALDI-TOF MS data analysis

Replicate m/z spectra data were generated for each lipid A. Bruker Daltonics flexAnalysis software was used to analyze all spectral data generated. Lipid A was classified as hexa- or hepta-acylated based on the m/z ratio of its primary spectral peak. On the basis of previous MS-based studies on LPS lipid A (35, 53–55), peaks with m/z ratios between 1650 and 1800 amu are predicted to be hexa-acylated, whereas peaks above 2000 amu are predicted to be hepta-acylated. Using these parameters, the acyl chain number present in lipid A of each strain was predicted. In addition, the loss or addition of a phosphate to the diglucosamine backbone was indicated by an m/z change of 80 amu, and the addition of PEtn was indicated by an m/z change of 123 amu. The presence of the outer membrane molecule, cardiolipin, was indicated by an m/z ratio value of 1456 amu and single carbon loss/additions.