Structure of Extracellular Glucans Synthesized by Streptococcus mutans of Serotypes a–e in vitro

Abstract

Insoluble extracellular glucans synthesized by Streptococcus mutans in vitro were separated from bacterial cells by solubilization in potassium hydroxide. After neutralization of the alkaline solution only part of the glucans precipitated giving fraction AS-I. Another part was isolated from the neutralized solution by the addition of ethanol yielding fraction AS-II. Soluble glucans harvested from the culture supernatant were treated in the same manner giving fractions WS-I and WS-II, respectively. By methylation analysis it was shown that the insoluble glucans of fraction AS-I are characterized by a high percentage of α-1,3-linked glucosyl residues and a low degree of branching. In contrast to this, soluble glucans of fraction WS-II represent highly branched polysaccharides with a content of α-1,6-linkages between 40 and 60%. Glucans of fraction AS-II originally present in the insoluble complex are significantly different from both fractions mentioned with respect to their percentages of α-1,3- and α-1,6-linked glucosyl residues. The linkage patterns of glucans of fraction WS-I are similar to those of fraction AS-I. Thus, with respect to the linkage patterns three distinct fractions of glucan were isolated from streptococcal cultures. A comparison of the glucans of the five serotypes revealed that glucans of type b, c, and e form one group of polysaccharides characterized by a greater mean percentage of α-1,6-linkages and branching sites than the other group consisting of glucans of type a and d. Furthermore, strains of S. mutans seem to be capable of synthesizing an almost continuous series of glucans with variable proportions of different types of linkages.