Enzymatic Galactosylation of Erythritol Enhances Antibacterial Activity against Cariogenic <i>Streptococcus mutans</i>

Erythritol is a widely used sweetener with beneficial properties and bioactivities, including the inhibition of Streptococcus mutans, a bacterium that induces dental caries. Galactosylation of compounds is an attractive method for improving antimicrobial activity. In this study, a novel metagenomic β-galactosidase gene, bga7, was successfully expressed in Escherichia coli. The recombinant enzyme was used to galactosylate erythritol, generating a high yield (93.6%) of galactoside product at a concentration of 2 U/mL upon incubation with 20 mM o-nitrophenyl-β-d-galactopyranoside and 0.5 M erythritol at 40 °C and pH 9.0 for 4 h. The product was confirmed to be β-galactosyl-erythritol by MS and NMR analysis. This galactoside demonstrated significantly enhanced inhibition of both the planktonic growth of S. mutans and biofilm formation compared to erythritol alone. Further investigation into the mechanism revealed that the galactoside suppressed the transcriptional levels of four important genes (gtfB, ftf, srtA, and spaP) associated with bacterial adhesion and biofilm formation.