Ultrastructural localization of sucrases in Streptococcus mutans GS-5 and an extracellular polysaccharide mutant: a comparative cytochemical and immunocytochemical study.

Electron microscopy and cytochemical and immunocytochemical procedures were used to study the ultrastructural distribution of sucrase enzymes in two strains of Streptococcus mutans. In a strongly adherent and virulent parent strain, GS-5, most of the invertase and fructosyltransferase activities were demonstrated extracellularly or bound to the cell surfaces. Intracellularly, enzymatic sites were detected near the plasma membrane on the periphery of the nucleoid and central mesosome. In GS-511, a mutant of diminished virulence and adherence, most of the enzymatic activity was not located on the cell surfaces, but was found away from the cell walls and associated with extracellular polysaccharides. Intracellularly, GS-511 manifested the same distribution of invertase and fructosyltransferase as did GS-5; however, the close association of these enzymes with the plasma membrane was not shown in GS-511. In both strains, extracellular areas near regions associated with cross wall formation appeared to show localized concentrations of these sucrases. Antibodies against partially purified glucosyltransferase (GTF) enzymes from GS-5 were used to localize GTF by immunocytochemical techniques. Indirect ferritin localization procedures showed that the extracellular and cell-bound GTF enzymes were distributed in similar locations as the fructosyltransferase and invertase enzymes. By absorption of the antiserum with whole GS-511 cells, the location of extracellular GTF and surface antigens unique to GS-5 was demonstrated. The dramatically reduced levels of cell-bound sucrase activity in GS-511 indicates the significant role of these enzymes in adherence and cariogenicity.