Genetic loci of Streptococcus mitis that mediate binding to human platelets

The direct binding of bacteria to platelets is a postulated major interacti on in the pathogenesis of infective endocarditis. To identify bacterial com ponents that mediate platelet binding by Streptococcus mitis, we screened a Tn916 DeltaE-derived mutant library of S, mitis strain SF100 for reduced b inding to human platelets in vitro. Two distinct loci were found to affect platelet binding. The first contains a gene (pblT) encoding a highly hydrop hobic, 43-kDa protein with 12 potential membrane-spanning segments, This pr otein resembles members of the major facilitator superfamily of small-molec ule transporters. The second platelet binding locus consists of an apparent polycistronic operon, This region includes genes that are highly similar t o those of Lactococcus lactis phage r1t and Streptococcus thermophilus phag e 01205. Two genes (pblA and pblB) encoding large surface proteins are also present. The former encodes a 107-kDa protein containing tryptophan-rich r epeats, which may serve to anchor the protein within the cell wall. The lat ter encodes a 121-M)a protein most similar to a tail fiber protein from pha ge 01205, Functional mapping by insertion-duplication mutagenesis and gene complementation indicates that PblB may be a platelet adhesin and that expr ession of PblB may be linked to that of PbIA. The combined data indicate th at at least two genomic regions contribute to platelet binding by S. mitis, One encodes a probable transmembrane transporter, while the second encodes two large surface proteins resembling structural components of lysogenic p hages.