Revisiting the endo and exo mode of action of dextran hydrolyzing enzymes, and their significance for Streptococcus mutans biofilm eradication.

Streptococcus mutans is a major microorganism causing dental caries. The mechanism by which caries arises involve the production of extracellular polysaccharides by S. mutans, which further assemble into a biofilm. Several strategies, including enzymatic deconstruction of the carbohydrate portion of the biofilm, can potentially be used to combat the formation of oral biofilms. We set out to heterologously produce two different S. mutans dextranases: DexA and DexB, from glycoside hydrolase families 66 (GH66) and 13 (GH13), respectively, and to study their capacity to degrade S. mutans biofilms. The S. mutans enzymes were recombinantly produced in E. coli, purified, biochemically characterized and applied to hydrolyze mature S. mutans biofilms as well as to prevent the biofilm formation. Two recombinant S. mutans dextranases have different modes of action: GH66 DexA is an endo-acting enzyme whereas DexB is an exo-acting enzyme. DexA is more thermostable than DexB, with the maximum activity observed at 40 °C and pH 5. When applied separately, DexA is capable of degrading S. mutans biofilms and displays synergism with α-1,3-glucanase, thereby further enhancing biofilm removal. Although DexB displayed a reduced potential to remove oral biofilms, when coupled with carbohydrate esterase from CE4 family and α-1,3-glucanase, biofilm removal above 50% was achieved. In addition to biofilm degradation, DexA inhibited biofilm formation, which was not observed for DexB. The results of this study can aid in the development of novel enzyme-based oral hygiene products.