Streptococcus salivarius strains carry either fibrils or fimbriae on the cell surface

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Abstract
Strains of S. salivarius were screened by negative staining for the presence of surface structures. Two structural subgroups were found, carrying either fibrils or fimbriae projecting from the cell surface. Eight strains carried a very dense peritrichous array of fibrils of 2 distinct lengths. Long fibrils had an average length of 175 nm; short fibrils had an average length of 95 nm. Two strains carried only long fibrils, 1 strain carried only short fibrils and another strain carried a lateral tuft of very prominent fibrils of 2 lengths, with a fibrillar fuzz covering the remainder of the cell surface. In all strains in which they were present, the long fibrils were unaffected by protease or trypsin treatment. The short fibrils were completely digested by protease and partially removed by trypsin. Neither long nor short fibrils were affected structurally by mild pepsin digestion or by lipase. The Lancefield extraction procedure removed both long and short fibrils. These 12 fibrillar strains were therefore divisible into 4 structural subgroups. Extracts of all the fibrilla strains reacted with group K antiserum. The 2nd main structural subgroup consisted of 9 strains of S. salivarius, which carried morphologically identical, flexible fimbriae arranged peritrichously over the cell surface. The fimbriae were structurally distinct from fibrils and measured 0.5-1.0 .mu.m long and 3-4 nm wide, with an irregular outline and no obvious substructure. There was no obvious reduction in the number of fimbriae after protease or trypsin treatment. Extracts of the fimbriated strains did not react with the group K antiserum. The 2 serological and structural subgroups could also be distinguished by colony morphology.