Involvement of sensor kinases in the stress tolerance response of Streptococcus mutans

The gram-positive bacterium Streptococcus mutans is the primary causative agent in the formation of dental caries in humans. The ability of S. mutans to adapt and to thrive in the hostile environment of the oral cavity suggests that this cariogenic pathogen is capable of sensing and responding to different environmental stimuli. This prompted us to investigate the role of two-component signal transduction systems (TCS), particularly the sensor kinases, in response to environmental stresses. Analysis of the annotated genome sequence of S. mutans indicates the presence of 13 putative TCS. Further bioinformatics analysis in our laboratory has identified an additional TCS in the genome of S. mutans. We verified the presence of the 14 sensor kinases by using PCR and Southern hybridization in 13 different S. mutans strains and found that not all of the sensor kinases are encoded by each strain. To determine the potential role of each TCS in the stress tolerance of S. mutans UA159, insertion mutations were introduced into the genes encoding the individual sensor kinases. We were successful in inactivating all of the sensor kinases, indicating that none of the TCS are essential for the viability of S. mutans. The mutant S. mutans strains were assessed for their ability to withstand various stresses, including osmotic, thermal, oxidative, and antibiotic stress, as well as the capacity to produce mutacin. We identified three sensor kinases, Smu486, Smu1128, and Smu1516, which play significant roles in stress tolerance of S. mutans strain UA159.