Evidence for crosstalk between the S. mutans glg and dlt loci

Streptococcus mutans is the major acidogenic component of dental plaque and the primary causative agent of tooth decay in humans. The S. mutans glg genes contribute to acid production and cariogenicity in the plaque environment by promoting the accumulation of glycogen-like storage polymers called intracellular polysaccharides (IPS). Interestingly, although IPS derive from expression of the S. mutans glg genes, an IPS-deficient mutant (SMS201) harbors a unique transposon insertion at the S. mutans dlt locus. The dlt genes have been characterized in other Gram-positive microorganisms where they are known to contribute to the D-alanylation of surface-associated teichoic and lipoteichoic acids. Such modification of bacterial surface molecules by D-alanine residues can ostensibly alter surface charge and ligand binding.

Objective: The main objective of this research is to elucidate the events at the S. mutans cell surface that influence IPS accumulation. Methods: To achieve this goal, we conducted RNA hybridization experiments, insertional mutagenesis, iodine staining, and hydrocarbon binding assays. Results: The results of these studies indicate that the S. mutans dlt and glg genes are coordinately regulated, and that a S. mutans mutant (GMS1000) that harbors a knockout mutation in the dlt operon is IPS-deficient. We also demonstrated decreased expression of the S. mutans glg genes in this mutant relative to the UA130 wild-type progenitor, and revealed that GMS1000 is altered in its surface hydrophobicity. Conclusion: Taken collectively, these data support the regulated expression of the S. mutans glg genes via a putative dlt-mediated signal transduction pathway. We are currently generating transcriptional fusions to further investigate crosstalk between the dlt and glg loci in this oral pathogen. In addition, we are growing S. mutans in biofilms to determine whether surface hydrophobicity affects IPS accumulation in the plaque environment. This research was supported by NIH grant # DE12306 (R29) and Middlebury College.