Role of Streptococcus gordonii amylase-binding protein a in adhesion to hydroxyapatite, starch metabolism, and biofilm formation

Interactions between bacteria and salivary components are thought to be imp ortant in the establishment and ecology of the oral microflora. oz-Amylase, the predominant salivary enzyme in humans, binds to Streptococcus gordonii , a primary colonizer of the tooth. Previous studies have implicated this i nteraction in adhesion of the bacteria to salivary pellicles, catabolism of dietary starches, and biofilm formation. Amylase binding is mediated at le ast in part by the amylase-binding protein A (AbpA). To study the function of this protein, an erythromycin resistance determinant [erm(AM)] was inser ted within the abpA gene of S. gordonii strains Challis and FAS4 by allelic exchange, resulting in abpA mutant strains Challis-E1 and FAS4-E1. Compari son of the wild-type and mutant strains did not reveal any significant diff erences in colony morphology, biochemical metabolic profiles, growth in com plex or defined media, surface hydrophobicity, or coaggregation properties. Scatchard analysis of adhesion isotherms demonstrated that the wild-type s trains adhered better to human parotid-saliva- and amylase-coated hydroxyap atite than did the AbpA mutants. In contrast, the mutant strains bound to w hole-saliva-coated hydroxyapatite to a greater extent than did the wild-typ e strains. While the wild-type strains preincubated with purified salivary amylase grew well in defined medium with potato starch as the sole carbohyd rate source, the AbpA mutants did not grow under the same conditions even a fter preincubation with amylase. In addition, the wild-type strain produced large microcolonies in a flow cell biofilm model, while the abpA mutant st rains grew much more poorly and produced relatively small microcolonies. Ta ken together, these results suggest that AbpA of S. gordonii functions as a n adhesin to amylase-coated hydroxyapatite, in salivary-amylase-mediated ca tabolism of dietary starches and in human saliva-supported biofilm formatio n by S. gordonii.