Streptococcus gordonii type VII secretion system substrate EsxA induces neutrophil extracellular trap formation in infective endocarditis

Type VII secretion system (T7SS) substrate EsxA contributes to the pathogenesis of several Gram-positive bacteria, but its role in viridans streptococci-induced infective endocarditis (IE) remains unclear. Genomic analysis of the Streptococcus gordonii DL1 strain identified a genetic locus encoding five T7SSb core machinery proteins (EsaA, EssA, EsaB, EssB, and EssC) and one substrate, EsxA. T7SSb mediates EsxA secretion, as evidenced by reduced secretion in the essC-deleted mutant and restoration upon complementation. In a rat model of IE, the esxA-deficient strain shows lower survival in circulation and reduced formation of vegetation, suggesting a role for EsxA in IE pathogenesis. Using a transwell system, molecules secreted from wild-type S. gordonii DL1 can induce the release of neutrophil extracellular traps (NETs) by neutrophils, and this activity is lost in the esxA-deficient strain. In addition, recombinant EsxA can dose-dependently induce NET formation, confirming a role for secreted EsxA in the induction of NET formation. Consistently, reduced and restored NET formation is observed in vivo in vegetations caused by esxA-deficient and complemented strains, respectively. Together, these data indicate that EsxA secretion mediated by T7SSb induces NET formation that contributes to bacterial survival in the circulation and vegetation formation in S. gordonii-induced IE.