Role of protease maturation lipoprotein in osmoadaptation of Streptococcus mutans.

Osmoadaptation may be an important trait for the pathogenicity of Streptococcus mutans . However, how this organism adapts to changes in osmolality in the oral cavity remains unclear. In this study, we showed that S. mutans utilizes K+ for osmoadaptation, in which protease maturation lipoprotein (PrtM) plays an important role. Although growth of the wild-type strain was impaired in a hyperosmotic medium [brain heart infusion (BHI) containing 0.3 M NaCl] compared with that in an unmodified BHI, the prtM mutant grew much more poorly in 0.3 M NaCl BHI. Comparison of growth behavior in the hyperosmotic medium supplemented with different osmoprotectants revealed that only the addition of K+ allowed the bacteria to overcome the impairment of growth caused by the high osmolality. These results suggest that K+ is an important compatible solute for S. mutans . Moreover, K+-associated recovery of growth was not observed for the prtM mutant, indicating that PrtM plays a critical role in the utilization of K+. Quantitative reverse-transcriptase polymerase chain reaction analysis showed that prtM was induced by osmotic stress, implying that prtM is an osmoresponsive gene. These findings suggest that K+ is an important compatible solute for S. mutans , and that the osmoresponsive lipoprotein PrtM is involved in K+ utilization, contributing to osmoadaptation of S. mutans .