Exploring the limits of bacterial identification by intact cell-mass spectrometry

The limits of intact cell-mass spectrometry (ICM-MS) were tested with regard to the minimum number of bacterial cells detectable and its power to discriminate mixed-bacterial cultures. The technique is a surface analysis tool, as is supported by evidence showing that mass fingerprints correspond to material desorbed directly from the cell wall. The brief exposure to solvents, which occurs during sample preparation, does not extract internal cellular material. Spectra were collected over the m/z range of 500 to 10,000. The UV absorbing matrices used were found to be highly specific to bacterial gram type: alpha-cyano-4-hydroxycinnamic acid for gram-negative bacteria and 5-chloro-2-mercaptobenzothiazole for gram-positive bacteria. This specificity allows mixed cultures of different gram types to be differentiated by ICM-MS. The minimum number of cells that could reliably give spectra of sufficient data was 10(4) cells (10(7) cells/mL).