Discrimination of some Gram-negative bacteria by direct probe mass spectrometry

The technique of direct probe mass spectrometry is described together with its application to the analysis of 50 strains of Gram-negative bacteria representing seven genera. Thirty-six of these strains were analysed in duplicate, and the 72 spectra held in a computer library. The intensities of 63 ions from each of the spectra were analysed by discriminant analysis techniques and all seven groups could be distinguished by as few as six ions. A set of 29 strains, 15 of which already in the data base, were used as unknowns to challenge the library on two separate occasions. The success rate of these challenges was 97% and 90% using the full spectra, but only 72% and 62% using the selection of six ions. Possible explanations for this are discussed as well as the scope and limitations of the method as a means of characterizing micro-organisms.