Mechanism of cell surface expression of the Streptococcus mitis platelet binding proteins PblA and PblB

Abstract

SummaryPblA and PblB are prophage-encoded proteins of Streptococcus mitis strain SF100 that mediate binding to human platelets. The mechanism for surface expression of these proteins has been unknown, as they do not contain signal sequences or cell wall sorting motifs. We therefore assessed whether expression of these proteins was linked the lytic cycle of the prophage. Deletion of either the holin or lysin gene resulted in retention of PblA and PblB in the cytoplasm, and loss of these proteins from the cell wall. Flow cytometric analysis revealed that induction of phage replication in SF100 produced a subpopulation of cells with increased permeability. This effect was abrogated by disruption of the holin and lysin genes. Treatment of these mutants with exogenous PblA and PblB restored surface expression, apparently via binding of the proteins to cell wall choline. Loss of PblA and PblB expression was associated with decreased platelet binding in vitro, and reduced virulence in an animal model of endocarditis. Thus, expression of PblA and PblB occurs via a novel mechanism, whereby phage induction increases bacterial permeability and release of the proteins, followed by their binding to surface of viable cells. This mechanism may be important for endovascular infection.

Metagenomic detection of phage-encoded platelet-binding factors in the human oral cavity

“…Phage SM1 is a temperate phage previously isolated from S. mitis SF100, an endocarditis strain (24). SM1 carries two genes, pblA and pblB, which contribute to S. mitis virulence in the endocardium (24)(25)(26)(27). Although S. mitis is a ubiquitous member of the normal oral flora, the presence of phage SM1 has never previously been reported in the mouth or oropharynx (1).…”

“…pblA and pblB are integral phage tail proteins, and phage with pblA and pblB gene deletions have intact capsids but no tails (24,25). pblA and pblB also mediate the attachment of S. mitis to platelets, which has been shown in a rabbit model to contribute significantly to the virulence of S. mitis in the endocardium (25)(26)(27)39). The interaction between S. mitis cells and platelets requires phage induction for maximal release of intracellular pblA and pblB, but the soluble proteins can bind to choline residues on the surface of host or nonhost cells (27,39).…”

“…pblA and pblB also mediate the attachment of S. mitis to platelets, which has been shown in a rabbit model to contribute significantly to the virulence of S. mitis in the endocardium (25)(26)(27)39). The interaction between S. mitis cells and platelets requires phage induction for maximal release of intracellular pblA and pblB, but the soluble proteins can bind to choline residues on the surface of host or nonhost cells (27,39). Theoretically, pblA and pblB genes inserted into any phage capable of host cell permeabilization or lysis would be sufficient to mediate S. mitis adhesion to platelets.…”

Characterization of Streptococcus gordonii prophage PH15: complete genome sequence and functional analysis of phage-encoded integrase and endolysin

“…Temperate phages from Streptococcus mitis, an abundant inhabitant of the oral cavity, have been described (Romero et al, 2004a;Siboo et al, 2003). S. mitis phage SM1 has been found to encode two proteins, PblA and PblB, which mediate binding to platelets and affect virulence in a rat model of infective endocarditis (Mitchell et al, 2007;Siboo et al, 2003).…”

Infective Endocarditis

“…Other proposed virulence factors that mediate streptococcal adhesion include FimA, which is a surface protein that functions as an adhesin in the oral cavity 43,44 , the sialic acid-binding adhesin Hsa 45 and a phage-encoded bacterial adhesin that mediates a complex interaction between bacteria, fibrinogen and platelets [46][47][48] .…”