Involvement of a predicted murein hydrolase in Streptococcus mutans' aciduricity

Background: Streptococcus mutans plays a major etiological role in the formation of dental caries due to acid production and a remarkable capacity to tolerate low pH, which can be detrimental to other oral bacteria. In order to sense and adapt to environmental stresses, bacteria are equipped with two component signal transduction systems (TCSTSs). In S. mutans, the VicK/R is one such system that comprises a membrane-bound sensor kinase and its response regulator, respectively. A microarray study using S. mutans UA159 wild-type and VicK knockout mutant strains allowed us to identify gene transcripts differentially regulated when cells were exposed to low pH, thereby supporting a regulatory role for the VicR/K TCSTS in the acid tolerance response (ATR) of S. mutans. Of these, a hypothetical protein designated Smu.574c containing an LrgB pfam domain, which has predicted murein hydrolase activity was selected for further study. Objective: To investigate the Smu.574c gene as part of the VicK regulon's involvement in the ATR of S. mutans. Method: A non-polar deletion mutant of Smu.574c was constructed using PCR mediated ligation mutagenesis, and its growth kinetics was determined under varying pH. Quantitative real-time PCR was used to study expression levels of Smu.574c in both UA159 and VicK-mutant strains using cells stressed under acid (pH 5.5) for 1 h. Cells exposed to pH 7.5 were used as controls. Results: Expression analyses revealed that Smu.574c was only induced under low pH and in the absence of VicK, thus suggesting a possible VicK-mediated negative regulatory role in Smu.574c transcription. Growth kinetics of the Smu.574c mutant did not reveal a dramatic difference in growth under low pH relative to wild type. Conclusions: Smu.574c is part of the VicK regulon that responds to acidic stress in S. mutans.  Acknowledgements: NIH Grant R01DE0132320 and CIHR Grant MT-15431.