Adherence of oral 'Streptococcus milleri' cells to surfaces in broth cultures

Abstract

Cells of representative strains of oral 'Streptococcus milleri' firmly adhered to glass surfaces when grown in glucose broths as well as in fructose and sucrose broths. Cellular adherence occurred on saliva-coated glass surfaces as well as uncoated surfaces, but the cells of only a few (6 of 69) tested adherent strains agglutinated upon the addition of human whole-saliva supernatant. Thus, the firm adherence of growing 'S. milleri' cells is independent of sucrose and salivary macromolecules. Two macroscopic forms of cell adherence were observed: one was a coarse-granule type and one was a tiny-granule type covered with a thin homogeneous membranous material. Scanning electron microscopy revealed that, in both types, a stringy substance interconnected the cells and connected the cells to glass surfaces, with amorphous flocklike materials present in the intercellular space. Of the 154 tested strains, 128 were adherent, of which 90 were firmly and extensively (+2 or +3) adherent. Most strains of the serovars f/F, g, h, i, j, g-, and g,j- were extensively adherent. Generally, cells of the more strongly adherent strains were more hydrophobic and spontaneously aggregating. However, there also existed a few hydrophilic and nonaggregating but extensively adherent strains. The presence of trypsin or pronase completely inhibited the adherence of some strains (mostly nonaggregating) but did not at all inhibit that of others (mostly aggregating). The adhering groups did not significantly differ in their cell surface hydrophobicities. Therefore, at least two factors, one proteinaceous and one nonproteinaceous, appear to be principally involved in the firm surface adherence of 'S. milleri.'