Exploring Novel Probiotic Mechanisms of Streptococcus A12 with Functional Genomics

Objectives: Health-associated commensals can promote oral health by fostering an environment that is less favorable for pathogens, such as Streptococcus mutans. Streptococcus A12 potently interferes with growth and virulence-related properties of S. mutans. Here, we sought a more complete understanding of the molecular mechanisms used by A12 to compete with S. mutans.
Methods: The interactions of S. mutans UA159 with A12 or A12 derivatives carrying defined mutations in three distinct genetic loci were examined under various conditions in different model systems. Mixed-species biofilms of A12 and S. mutans expressing fluorescent proteins were imaged with confocal laser scanning microscopy, analyzed using COMSTAT and CFU were quantified. Transcriptional organization and regulation of gene expression was assessed with 5’-RACE and RT-qPCR to gain insights into their function.
Results: The genes of interest were shown to contribute to the tolerance and the antagonism of S. mutans. Differences in biofilm architecture and composition were evident when comparing dual-species biofilms formed by S. mutans and A12 with those formed by S. mutans and the A12 mutants. The gene products were shown to affect competition in distinct ways: i) by interfering with intercellular communication pathways of S. mutans; ii) by sensing, and conferring resistance to, antimicrobial peptides; and iii) by participating in biogenesis of a factor that antagonizes the growth of S. mutans.
Conclusions: Our current understanding of the intermicrobial interactions and probiotic mechanisms of beneficial commensals that shape the composition and pathogenic potential of oral biofilms is rudimentary. This study demonstrates that the ability of commensal oral streptococci to compete effectively with S. mutans involves multiple gene products with a spectrum of functions. Knowledge gained here and with additional functional genomic studies using other commensal streptococci will accelerate the development of more-effective risk assessment, prevention and therapeutic strategies to control dental caries and other oral infectious diseases.