Cellobio-oligosaccharides (CBO) produced by the dextransucrase-catalyzed transglycosylation reaction of sucrose and cellobiose were discovered to be effective as antifungal agents against dental caries and against fungi who rely on glucan as an integral part of the cell wall, e.g., A. terreus. The cellobio-oligosaccharides were found to be inhibitors of .beta.-(1,3)-glucan synthase, an important enzyme involving in fungal cell wall component synthesis. The CBO caused structural changes in the growing fungal cells. In addition, the CBO were shown to be effective as anti-cariogenic agents in preventing bacterial adherence to teeth by inhibiting the formation of the bacterial plaque (glucans), e.g., that formed by Streptococcus mutans. Cellobio-oligosaccharides produced by dextransucrase were analyzed and shown to have a degree of polymerization (DP) ranging from 3 to 6 glucosyl groups. Examples of these cellobio-oligosaccharides produced by this method include, but are not limited to, trisaccharides such as .alpha.-D-glucopyranosyl-(1.fwdarw.2)-.beta.-D-glucopyranosyl-(1.fwdarw.4- )-D-glucopyranose and .alpha.-D-glucopyranosyl-(1.fwdarw.6)-.beta.-D-glucopyranosyl-(1.fwdarw.4- )-D-glucopyranose.
 This invention was made with the United States government support under contract No. DE-FG36-04GO14236 awarded by the Department of Energy. The United States government has certain rights in this invention.