Ultrastructure of Mutants of Streptococcus mutans with Reference to Agglutination, Adhesion, and Extracellular Polysaccharide.

An electron microscopic study of S. mutans 6715-13 wild type and representatives of three distinct classes of glucan synthesis-defective mutants (which fail to form adherent microbial plaques but agglutinate normally in the presence of exogenous glucans) disclosed the presence of two sucrose-dependent, glucanase-sensitive, extracellular components. In the wild type, these extracellular glucans had predominantly fibrillar (with some globular) morphologies. However, in the mutant strains, there was a consistent reduction in or loss of the fibrillar components and dramatic increases in globular forms. A cell surface-associated fuzzy coat was consistently seen, and it was neither sucrose-dependent nor glucanase-sensitive. The data indicated that in vitro and in vivo adhesion and virulence at smooth tooth surfaces (all these properties dramatically reduced in the mutants) were causally and functionally related to the extracellular, fibrillar, glucan component, whereas in vitro glucan-mediated agglutination may be related to the cell-associated surface fuzzy coat.