Outer membrane vesicle-mediated serum protection in<i>Aggregatibacter actinomycetemcomitans</i>

Abstract

<i>Aggregatibacter actinomycetemcomitans</i> belongs to the HACEK group of fastidious Gram-negative organisms, a recognized cause of infective endocarditis. <i>A. actinomycetemcomitans</i> is also implicated in periodontitis, with rapid progress in adolescents. We recently demonstrated that the major outer membrane protein, OmpA1 was critical for serum survival of the <i>A. actinomycetemcomitans</i> serotype a model strain, D7SS, and that the paralogue, OmpA2 could operate as a functional homologue to OmpA1 in mediating serum resistance. In the present work, an essentially serum-sensitive <i>ompA1 ompA2</i> double mutant <i>A. actinomycetemcomitans</i> strain derivative was exploited to elucidate if <i>A. actinomycetemcomitans</i> OMVs can contribute to bacterial serum resistance. Indeed, supplementation of OMVs resulted in a dose-dependent increase of the survival of the serum-sensitive strain in incubations in 50% normal human serum (NHS). Whereas neither OmpA1 nor OmpA2 was required for the OMV-mediated serum protection, OMVs and LPS from an <i>A. actinomycetemcomitans</i> strain lacking the LPS O-antigen polysaccharide part were significantly impaired in protecting D7SS <i>ompA1 ompA2</i>. Our results using a complement system screen assay support a model where <i>A. actinomycetemcomitans</i> OMVs can act as a decoy, which can trigger complement activation in an LPS-dependent manner, and consume complement components to protect serum-susceptible bacterial cells.