Cluster Analysis of Bacteria Basedon Data Obtained from Intact CellMatrix Assisted Laser Desorption/Ionisation Time of Flight Mass Spectrometry

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Brigh, J.J[1];Sutton, H.E[1];Edwards-Jones, V[1];Dare, D.J[1];Keys, C.J[2];Shah, H[2];McKenna, T[3];Wells, G[3];Lunt, M[3]
ASM 2002; Salt Lake City; Utah; 19th-23rd May; 2002
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Intact cell matrix assisted laser desorption/ionisation time of flight mass spectrometry (MALDI-TOF-MS) also known as ICM-MS produces characteristic mass spectral fingerprints of moieties desorbed from the bacterial surface. Mass spectra are obtained within minutes and we have acquired the fingerprints of ~1000 NCTC strains of bacteria in a searchable database. Matching ICM-MS spectra of unknown bacteria to this database provides a rapid new method of identification of bacteria. The relatedness of the mass spectral fingerprints of bacteria in the database can also be determined by cluster analysis of this data. A software package has been written specifically to clust ICM-MS data. The promimity of one bacterium to another was generated by using a spectral RMS calculation and grouping clusters of bateria by simple average proximity. In this study the cluster package was tested firstly, to determine the relationship of known bacteria based on their mass spectral fingerprints with their relationship based on more traditional methods and secondly, to determine the level of discrimination that is possible by clustering ICM-MS data. In an experiment designed to test the cluster analysis package twelve strains of bacteria covering a range of relatedness from Gram-positive to Gram-negative, to two different strains of Escherichia coli (both 011:K58(B4):H2) and two strains of Staphylococcus aureus (the Type strain and EMRSA-16), demonstrated that clusterin was able to resolve the two strains of these species. With a few notable exceptions, the relationship of the strains shown in the dendrograms of the ICM-MS data was what might be expected from their accepted relationships e.g. Escherichia coli strains were more closely related to each other than Escherichia hermanii and more closely related to each other than to Citrobacter freundii. However there were some exceptions. Streptococcus pyogenes clustered more closely to Pseudomonas aeruginosa and all but on of the other Gram-negative bacteria, than to the other Gram-positive bacteria. These results reflect similarities in the moieties desorbed from the surface of these bacteria and therefore, demonstrate relationships that are not always apparent by other methods. The cluster analysis has been applied to groups of bacteria already in the database of ~1000 strains revealing some intriguing results that may help to elucidate the nature of the moieties ionised from the surface of the bacteria. The five O157:H7:VT-strains, of all the 28 strains of Escherichia coli in the database, all clustered closely together indicating that the O157 antigen may be the major moiety that contributes to the mass spectral fingerprint of these strains. In a similar way, cluster analysis of other groups in the database may give us further clues to the nature of the surface moieties involved in producing ICM-MS spectra

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