Methods: Strep.- mutans-biofilms were grown on human enamel slides for 24h in microtiter plates. The biofilm formation was monitored under three different dietary conditions: control medium containing glucose (A), medium A supplemented with 5% sucrose (B) and medium A with 1% xylitol (C). The gene expression of glucosyltransferases (GtfB, GtfC, GtfD, GtfA), fructosyltransferase (Ftf) and other carbohydrate-associated genes of biofilm-associated Strep. mutans cells were analysed using RT-qPCR. Complementary total bacterial cell counts (BC) and colony forming units (CFU) of streptococci grown in biofilms and pH of the environmental solutions were investigated. Statistical analysis of the normally distributed data was performed (t-test, α = 0.05).
Results: Compared to sucrose supplement, the gene expression of water-insoluble and soluble glucans producing GtfB/C were upregulated in the presence of xylitol, while the gene expression of only water-soluble glucans producing GtfD and Ftf showed no significant difference and the one of GtfA was downregulated. Concerning microbial parameters significant differences could be observed between the reduced BC deposited on enamel slides under xylitol supplement compared to BC of biofilms in media A and B. The pH-values of the three growth media differed significantly showing the lowest value in medium B (pH 4.28), and increasing values in medium C (pH 4.40) and A (pH 4.54).
Conclusions: Unexpectedly, in Strep.-mutans-biofilms supplemented with xylitol, the expression of gtfB/C genes, responsible for water-insoluble glucan synthesis was significantly higher than the gene expression in biofilms supplemented with sucrose. These genetic activities may be an attempt to counterbalance the adverse effects of xylitol.