Biology of Streptococcus mutans-derived glucosyltransferases: role in extracellular matrix formation of cariogenic biofilms

Abstract

The importance of Streptococcus mutans in the etiology and pathogenesis of dental caries is certainly controversial, in part because excessive attention is paid to the numbers of S. mutans and acid production while the matrix within dental plaque has been neglected. S. mutans does not always dominate within plaque; many organisms are equally acidogenic and aciduric. It is also recognized that glucosyltransferases from S. mutans (Gtfs) play critical roles in the development of virulent dental plaque. Gtfs adsorb to enamel synthesizing glucans in situ, providing sites for avid colonization by microorganisms and an insoluble matrix for plaque. Gtfs also adsorb to surfaces of other oral microorganisms converting them to glucan producers. S. mutans expresses 3 genetically distinct Gtfs; each appears to play a different but overlapping role in the formation of virulent plaque. GtfC is adsorbed to enamel within pellicle whereas GtfB binds avidly to bacteria promoting tight cell clustering, and enhancing cohesion of plaque. GtfD forms a soluble, readily metabolizable polysaccharide and acts as a primer for GtfB. The behavior of soluble Gtfs does not mirror that observed with surface-adsorbed enzymes. Furthermore, the structure of polysaccharide matrix changes over time as a result of the action of mutanases and dextranases within plaque. Gtfs at distinct loci offer chemotherapeutic targets to prevent caries. Nevertheless, agents that inhibit Gtfs in solution frequently have a reduced or no effect on adsorbed enzymes. Clearly, conformational changes and reactions of Gtfs on surfaces are complex and modulate the pathogenesis of dental caries in situ, deserving further investigation.

Fig. 1

a Representative confocal images of bacterial cells (in green) and glucans (in red) within biofilms formed by S. mutans US 159 on tooth enamel surface in the presence of 1% (wt/vol) sucrose. Arrowheads indicate structural organization of glucans within microcolonies. b Close-up view of 3-dimensional structural relationship between glucans and S. mutans cells.

Fig. 2

Revised model of Gtf-glucan-mediated bacterial adherence and cariogenic biofilm development. Originally proposed by Rölla et al. [1983b]. (1) The Gtfs secreted by S. mutans are incorporated into pellicle (particularly GtfC) and adsorb on bacterial surfaces (mainly GtfB), including microorganisms that do not produce Gtfs (e.g. Actinomyces spp.). Furthermore, salivary α-amylase is also included into pellicle, which can also bind Gtfs. (2) Surface-adsorbed GtfB and GtfC rapidly utilize dietary sucrose to synthesize insoluble and soluble glucans in situ; the soluble glucans formed by GtfD could serve as primers for GtfB enhancing the overall synthesis of exopolysaccharides. Concomitantly, starch is digested by amylase releasing maltose and a myriad of oligosaccharides; they can be incorporated into the polymer molecule through acceptor reactions, particularly by surface-adsorbed GtfB. The Gtfs adsorbed onto enamel and microbial surfaces provide in situ an insoluble matrix for dental plaque. (3) The glucan molecules provide avid binding sites on surfaces for S. mutans (and other microorganisms) mediating tight bacterial clustering and adherence to the tooth enamel. Furthermore, Gtf-adsorbed bacteria become de facto glucan producers binding to tooth and microbial surfaces by the same mechanisms. This model could explain the rapid formation and accumulation of highly cohesive-adherent plaque in the presence of sucrose (and possibly starch) even if the number of S. mutans is relatively low. After the establishment of a glucan-rich biofilm matrix, ecological pressure (e.g. pH) will determine which bacteria may survive and dominate within plaque under frequent sucrose (or other fermentable carbohydrate) exposure.