Structural basis for the roles of starch and sucrose in homo-exopolysaccharide formation by Lactobacillus reuteri 35-5

• Many Lactobacillus have both glucansucrase and 4,6-α-glucanotransferase genes.
• L. reuteri 35-5 secretes both glucansucrase GtfA and 4,6-α-glucanotransferase GtfB.
• GtfA and GtfB contribute to exopolysaccharide (EPS) formation by L. reuteri 35-5.
• GtfA and GtfB cross-react in hybrid EPS synthesis from sucrose and starch.

Lactic acid bacteria (LAB) produce exopolysaccharides (EPS) that are important for biofilm formation in the mammalian oral cavity and gastrointestinal tract. Sucrose is a well-known substrate for homo-EPS formation by Lactobacillus reuteri glucansucrases (GS). Starch is the main fermentable carbohydrate in the human diet, and often consumed simultaneously with sucrose. Recently we have characterized L. reuteri strains that also possess 4,6-α-glucanotransferases (4,6-α-GTases) that act on starch yielding isomalto-/malto-polysaccharides. In this study we have characterized the EPS formed by L. reuteri 35-5 cells and enzymes from sucrose plus starch. The data show that both in vivo and in vitro the L. reuteri 35-5 GS and 4,6-α-GTase enzymes, incubated with sucrose plus starch, cross-react and contribute to synthesis of the final hybrid EPS products. This may have strong effects on the EPS functional properties, influence biofilm formation, and affect the relationship between dietary intake of sucrose and starch, and dental caries formation.