Phenotypic Characterization of Genetically-Diverse Clinical Isolates of Streptococcus mutans

Objective: High coverage genome sequencing of 57 isolates of Streptococcus mutans was recently completed.  Fifteen genetically heterogeneous strains were analyzed for sensitivity to stress and competence stimulating peptide (CSP), biofilm formation, and autolysis.

Methods: Growth was monitored in BHI broth at pH 7.0 or 5.5, in aerobic or anaerobic conditions, or in the presence of CSP or 25mM paraquat.  Autolysis assays were performed with cells from late exponential phase cultures.  Biofilm formation was monitored in BM medium with 20 mM glucose or sucrose in a microtiter plate assay.  

Results: A broad spectrum of tolerance to environmental stressors and capacity to form biofilms was observed.  For example, Smu86 was unusually sensitive to growth inhibition by CSP and displayed enhanced genetic competence.  Smu21 and Smu44 demonstrated high constitutional resistance to acid, whereas Smu86 and Smu56 were more sensitive to acid stress. Smu81 did not grow, and Smu21 and Smu86 grew much better than all other strains, in paraquat.  Biofilm formation in most strains was enhanced in sucrose compared to glucose, although Smu56 and Smu57 formed biofilms equally well in both sugars, and Smu77 formed very poor biofilms under all conditions. 

Conclusion: Despite occupying a specialized niche in the oral cavity, our analysis highlights the remarkable diversity of S. mutans and suggests the use of a variety of strategies and gene products to persist in the oral cavity.  A comparison of the core and non-core gene content of these strains is ongoing to correlate with virulence-related phenotypes. Also, an analysis of the transcriptome of these distinct isolates is underway to shed light on adaptive strategies and to provide additional explanations for phenotypic diversity.  Collectively, these studies will provide new knowledge that can guide the development of more broadly-effective therapies against this caries pathogen.