Surface structures (peritrichous fibrils and tufts of fibrils) found on Streptococcus sanguis strains may be related to their ability to coaggregate with other oral genera.

We screened 36 strains of Streptococcus sanguis biotype I and 8 strains of S. sanguis biotype II for the presence of surface structures and for their ability to coaggregate with Actinomyces viscosus, Actinomyces naeslundii, and Fusobacterium nucleatum. Negative staining under an electron microscope revealed detectable surface structures on all S. sanguis strains. The majority of strains (38 of 44) carried peritrichous fibrils, which have an irregular profile and no distinct width. They usually appeared as a fringe with a constant width around the cell. Strains selected for measurement had a fringe with an average length of 72.4 +/- 8.5 nm on biotype I strains and 51.6 +/- 3.3 nm on biotype II strains. Some fibrillar biotype I strains carried an additional, longer (158.7 +/- 33.1 nm) type of fibril projecting through the shorter fibrils. Fibrillar density was characteristic for each strain, ranging from very dense on all cells in a population to very sparse on a few cells in a population. A small group of six strains carried tufts of fibrils in a lateral or polar position on the cell. Either one or two lengths of fibril were present in the tuft depending on the strain. One strain carried both peritrichous fibrils and fimbriae. Fimbriae are flexible structures with a constant width (4.5 to 5.0 nm) all along their length but very variable lengths (less than or equal to 0.7 micron) on each cell. S. sanguis I and II both included strains with peritrichous fibrils and tufts of fibrils, but the mixed morphotype strain was confined to biotype II. Fibrils were present on cells at all stages throughout the growth cycle for the strains tested. Freshly isolated fibrillar strains coaggregated consistently well with A. viscosus and A. naeslundii, although some fibrillar reference strains lacked the ability. In addition, all tufted strains could not coaggregate, but the strains with the mixed morphotype coaggregated well. Coaggregation with F. nucleatum was very strong for the fibrillar strains, but less strong for the tufted strains. We discuss the possible correlation between S. sanguis surface structure and ability to coaggregate.