Proteins secreted by Streptococcus mutans

Extracellular proteins in conjunction with cell-associated components has been proposed to make a major contribution to the virulence in many different bacteria. Our previous research with Streptococcus mutans has indicated that extracellular proteins may be involved in the induction of an acid tolerance response, resulting in increased survival at low pH (3.5-3.0). Objectives: This work aims at investigating whether S. mutans exports proteins at high cell densities, conditions resembling that in a biofilm where close cell-to-cell contact allows intercellular signalling. Methods: Log-phase cells were concentrated 100 times before being inoculated into a complex medium containing [¹⁴C] amino acid mixture and with different nutrient levels and pH. These were then incubated anaerobically at 37°C for 2 hours and ten times concentrated culture supernatants were analysed by two-dimensional gel electrophoresis followed by autoradiography. Protein spots were excised from duplicates of colloidal Coomassie blue stained gels and subjected to tryptic digestion. The peptide extract was analysed by MALDI-TOF-MS and the proteins were identified by peptide mass fingerprints with the aid of the sequence database of S. mutans. Results: The secretion of proteins was found to be dependent on the presence of glucose and pH with most proteins secreted at pH 7.0. Non [¹⁴C]-labelled proteins also appeared in the medium indicating a proteolytic activity. Conclusions: S. mutans exports several different proteins during a two hour growth in a planktonic high cell density culture. As expected cell surface antigen I/II and glucosyltransferase were detected in the extracellular medium. Interestingly several of the observed proteins in the supernatant are usually associated with the intracellular compartment including trigger factor ppiase, DnaK, superoxide dismutase and phosphoglycerate kinase. Supported by The Swedish Research Council and The Knowledge Foundation, Sweden.