, an “obligate” biofilm-forming bacterium (5
) and the primary etiological agent of human dental caries, has developed a variety of mechanisms to colonize the tooth surfaces and, under certain conditions, to become a numerically significant species in cariogenic biofilms (4
). Moreover, this bacterium possesses the ability to scavenge multiple fermentable sugars at micromolar concentrations and to grow and carry out glycolysis at the low plaque pH values attained in the oral cavity. The production of acids by S. mutans
causes dissolution of minerals in tooth enamel and formation of dental caries.
Despite the complexity of molecular mechanisms implicated in the process, biofilm formation is generally considered to be a two-step sequential process requiring attachment of the bacterial cells to a surface (early adherence), followed by growth-dependent accumulation of bacteria in multilayered cell clusters, involving intra- and intergeneric adhesion events. Considerable effort has been devoted to identifying factors of S. mutans
and other oral streptococci that participate in biofilm initiation and development. Surface-associated proteins, such as SpaP and Fap1, have been found to function as high-affinity adhesins and play an important role in the initiation of biofilm formation by oral streptococci (2
). Extracellular glucans, especially the insoluble glucans synthesized from sucrose by the glucosyltransferases, are of central importance in adhesive interactions by mutans streptococci and are key factors in the accumulation of mutans streptococci on smooth surfaces (22
). Reduction in the synthesis of water-insoluble glucan through directed mutagenesis was found to result in decreased biofilm formation by S. mutans
and reduced cariogenicity in rat caries models (8
). S. mutans
also synthesizes three glucan-binding proteins: GbpA, GbpB, and GbpC. GbpC has similarities to members of the Spa family of proteins of oral streptococci and is involved in rapid, dextran-dependent aggregation (31
). Immunization with GbpB has been reported to be protective against caries (33
), although Hazlett et al. (18
) have reported that a gbpA-
deficient strain is hypercariogenic in a rat model. It was later found that inactivation of gbpA
alters the structural and functional aspects of plaque biofilm that could be compensated by recombination between the gtfB
). Thus, there are numerous cell surface and extracellular proteins that work in concert to successfully establish S. mutans
in tooth biofilms.
While much is now known about initial adhesion of S. mutans
to the tooth, very little is known about maturation of oral biofilms formed by this organism or about how the organism controls expression of virulence when it is growing as adherent populations. Importantly, it is generally believed that the regulation of gene expression in biofilm populations is different from that in planktonic cells (10
) and that the bacteria must coordinate the expression of sets of genes to form stable biofilms (9
). Here, we report the identification and characterization of a gene that was found to encode a novel protein that appears to be involved in regulation of biofilm development by S. mutans
, and we explore the impact of inactivation of genes potentially involved in quorum sensing (QS) and in the control of carbohydrate metabolism on the ability of this bacterium to form biofilms.