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Genome sequence data provide a framework for predicting potential microbial activities; however, the proteome content of the cell dictates its response to its environment. Microbiology is witnessing a major initiative to elucidate the nature of the proteome of large numbers of species. The tool driving the proteomic revolution is matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. During the analysis process, proteins are ionized and separated on the basis of their mass-to-charge ratios, which results in a characteristic mass-spectral profile. Because of the dynamic nature of the cell and the large number of external parameters that could influence its mass-spectral profile, considerable work was needed initially to optimize sample analysis and obtain consistent and reproducible results. For many anaerobes that grow poorly or are nonreactive in most diagnostic systems, proteome analysis is likely to have a major impact on microbial diagnosis and the delineation of centers of diversity associated with infections.

Exiguobacterium spp. are alkaliphilic, halotolerant, non-spore-forming Gram-positive bacilli, hitherto uncharacterized from human infections. Six isolates of Exiguobacterium aurantiacum were obtained from patients with bacteraemia, three of whom had myeloma. All isolates formed orange-yellow pigmented colonies on blood agar, were catalase- and DNase-positive, and grew on nutrient agar at pH 10 and in the presence of NaCl 6% w/v. The six isolates were susceptible to all antimicrobial agents tested and were uniform in their fatty acid and mass spectrum profiles.

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Matrix-Assisted Laser Desorption:lonisation Time of Flight Mass Spectrometry (MALDI-TOF-MS) as a tool for differentiating bacterial species was examined using reference strains representing gram-positive and gram-negative taxa. Initially, the effect of differences in medium composition on spectral pro le was examined. The results indicated that growth on Columbia blood agar resulted in a larger spectrum of ionized residues and was therefore used for the cultivation of all strains in the rest of the study. The stability of the obtained mass spectral pro les against differences in batch and media processing suggested that no signi cant alterations to the pro les occurred in response to changes in media sources. The established conditions from these initial experiments were used to standardize subsequent experiments. The MALDI-TOF-MS pro les of 15 reference strains were compared and species characteristic markers were identi ed. The potential of using MALDI-TOF-MS as a tool for probing clonal diversity was examined using well characterized but clonally variable isolates of Bacteroides fragilis. Comparative analysis of the pro les of 20 strains revealed 5 clusters within the species but compared to other taxa such as Bacteroides merdae and Salmonella arizonae they form a closely related lineage. These results obtained strongly support the potential to use MALDI-TOF-MS as a tool for exploring bacterial surfaces for characteristic biomarkers and species-speci c signatures.