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Cellular and extracellular proteome analysis of Streptococcus mutans grown in a chemostat

The oral pathogen, Streptococcus mutans, was grown under glucose limitation in a chemostat at pH 7.0 and a dilution rate of 0.1 h−1 to mimic the conditions prevailing in a healthy human oral cavity in between meal times. Solubilized cellular and extracellular proteins were separated by two‐dimensional gel electrophoresis (2‐DE) and, following tryptic digestion, 421 protein spots analyzed by matrix‐assisted laser desorption/ionization‐time of flight mass spectrometry or electrospray ionization‐tandem mass spectrometry. Analyses of the mass spectral data showed that the proteins matched the translation products of 200 different open reading frames (ORFs) deduced from contigs of the S. mutans UA159 genome and thus represented proteins derived from approximately 11% of the total ORFs of the bacterium. Of the identified proteins, 172 (including one surface protein) were characterized in the cellular fraction, and the remaining 28 (including two surface proteins) were uniquely identified from the culture fluid. The expression and therefore the existence of 30 proteins previously designated as ‘hypothetical’ or with no known function was confirmed. 2‐DE of whole cell lysates revealed only a single intrinsic membrane protein. This is consistent with proteomic analyses of other Gram‐positive bacteria where hydrophilic proteins represent the vast majority of those characterized.