Identification of a functional capsule locus in Streptococcus mitis

The polysaccharide capsule of Streptococcus pneumoniae is a hallmark for virulence in humans. In its close relative Streptococcus mitis, a common human commensal, analysis of the sequenced genomes of six strains revealed the presence of a putative capsule locus in four of them. We constructed an isogenic S.similar to mitis mutant from the type strain that lacked the 19 open reading frames in the capsule locus (?cps mutant), using a deletion strategy similar to previous capsule functional studies in S.similar to pneumoniae. Transmission electron microscopy and atomic force microscopy revealed a capsule-like structure in the S.similar to mitis type strain that was absent or reduced in the ?cps mutant. Since S.similar to mitis are predominant oral colonizers of tooth surfaces, we addressed the relevance of the capsule locus for the S.similar to mitis overall surface properties, autoaggregation and biofilm formation. The capsule deletion resulted in a mutant with approximately two-fold increase in hydrophobicity. Binding to the Stains-all cationic dye was reduced by 40%, suggesting a reduction in the overall negative surface charge of the mutant. The mutant exhibited also increased autoaggregation in coaggregation buffer, and up to six-fold increase in biofilm levels. The results suggested that the capsule locus is associated with production of a capsule-like structure in S.similar to mitis and indicated that the S.similar to mitis capsule-like structure may confer surface attributes similar to those associated with the capsule in S.similar to pneumoniae.