INACTIVATION OF THE GBPA GENE OF STREPTOCOCCUS-MUTANS INCREASES VIRULENCE AND PROMOTES IN-VIVO ACCUMULATION OF RECOMBINATIONS BETWEEN THE GLUCOSYLTRANSFERASE-B AND GLUCOSYLTRANSFERASE-C GENES

Glucan-binding protein A (GbpA) of Streptococcus mutans has been hypot hesized to promote sucrase-dependent adherence and the cohesiveness of plaque and therefore to contribute to caries formation. We have analy zed the adherence properties and virulence of isogenic gbpA mutants re lative to those of wild-type S. mutans, Contrary to expectations, the gbpA mutant strains displayed enhanced sucrose-dependent adherence in vitro and enhanced cariogenicity in vivo. In vitro, S. mutans was grow n in the presence of [H-3] thymidine and sucrose within glass vials. W hen grown with constant rotation, significantly higher levels of gbpA mutant organisms than of wild type remained adherent to the vial walls . Postgrowth vortexing of rotated cultures significantly decreased adh erence of wild-type organisms, whereas the adherence of gbpA mutant or ganisms was unaffected. In the gnotobiotic rat model, the gbpA mutant strain was hypercariogenic though the colonization levels were not sig nificantly different from those of the wild type. The gbpA mutant stra in became enriched in vivo with organisms that had undergone a recombi nation involving the gtfB and gtfC genes. The incidence of gtfBC recom binant organisms increased as a function of dietary sucrose availabili ty and was inversely correlated with caries development. We propose th at the absence of GbpA elevates the cariogenic potential of S. mutans by altering the structure of plaque. However, the hypercariogenic plaq ue generated by gbpA mutant organisms may be suboptimal for S. mutans, leading to the accumulation of gtfBC recombinants whose reduced gluco syltransferase activity restores a less cariogenic plaque structure.