Characterization of the regulation of autolysin enzymes in the oral pathogen Streptococcus mutans: The importance of the LytR and LytST genes

Streptococcus mutans is the primary odontopathogen present in supragingival plaque, and causes the oral disease known as dental caries. Colonization of the oral cavity by Streptococcus mutans requires the bacteria to adhere to the tooth surface and occurs by both sucrose-dependent and sucrose-independent mechanisms. Sucrose-independent adhesion of S. mutans in vitro has been shown to involve an open reading frame (ORF0371) encoding a homolog (39%) to LytR, a regulator of autolysin activity in Bacillus subtilis. The protein encoded by ORF0371, LytR, belongs to the LytR/CpsA/Psr protein family. This family has a putative role in cell-wall structural maintenance, possibly through autolysin regulation. Autolysins have also been shown to be important in surface adhesion in Lactococcus lactis, and in the pathogenic properties of Streptococcus pneumoniae. The regulation of autolysins, enzymes that hydrolyze bacterial peptidoglycan, is not well understood in S. mutans . It has been hypothesized that the LytR protein is required for negative regulation of autolysin enzymes in S. mutans. To investigate the role of autolysins in the adhesion and pathogenesis of S. mutans, a LytR mutant was constructed. The mutant grows in long chains, which may indicate a defect in cell division. Further experiments with the mutant strain showed increased autolytic activity, and indicated that LytR attenuates S. mutans autolytic activity, possibly through regulation of the expression of autolytic enzymes. No defect in cell-to-surface adherence or biofilm growth was seen in the LytR mutant. Also, LytR negatively regulates the transcription of at least 2 autolysin genes of S. mutans , since the transcript abundance was increased in a LytR mutant strain. Also present in the S. mutans genome is a two-component regulatory system, LytST, a homolog of the well-characterized LytSR two-component system of Staphylococcus aureus. A LytST mutant strain was created to characterize the importance of these genes in the physiology and gene regulation of S. mutans. The LytST two-component system is also responsible for negative regulation of the transcription of putative autolysin enzymes in S. mutans, and required for the transcription of the LrgAB genes, which may function as anti-holins.