Streptococcus mutans: Quorum sensing within mixed biofilms

We have previously reported that the presence of other species of oral streptococci such as Streptococcus gordonii interferes with quorum sensing properties of Streptococcus mutans, including the acid tolerance response, natural transformation and bacteriocin production. All of these activities have been shown to be mediated by quorum sensing, which depends on a competence-stimulating peptide (CSP) signaling system that is encoded by the com genes.

Objective: To determine if early colonizers (S.  gordonii or Actinomyces naeslundii) antagonize colonization efficiency of S. mutans in sequential biofilms. Methods: i) Biofilm assays: S. gordonii or A. naeslundii were grown overnight in microtiter plates to form biofilms and S. mutans was added the next day to mimic the in vivo situation of sequential biofilms.  ii) Azocasein Assay: Cell pellets from S. gordonii or A. naeslundii were resuspended in 0.5% Azocasein. OD420 measured afterwards indicated the amount of protease present. iii) Measurement of S. mutans CSP inactivation by the early colonizers: The ability to inactivate S. mutans CSP by S. gordonii or A. naeslundii was assessed by incubating exogenous S. mutans CSP with S. gordonii or A. naeslundii. Results: S. gordonii antagonized the colonization of S. mutans, whereas A. naeslundii did not.  Subsequent analysis with assays such as Azocasein and CSP inactivation demonstrated that S. gordonii, not A. naeslundii, produces serine protease, which inactivated S. mutans CSP and thereby inhibited the attachment of S. mutans within the biofilm. Utilization of a protease knockout mutant of S. gordonii resulted in a significantly less antagonism of S. mutans colonization, relative to the wild-type S. gordonii. Conclusion: S. gordonii antagonizes the sequential biofilm attachment of S. mutans. The mechanism of this antagonism involves S. gordonii expression of a protease which interrupts the Com quorum sensing system in S. mutans. This work is supported in part by DE017708.