Role of OmpA1 and OmpA2 in <i>Aggregatibacter actinomycetemcomitans</i> and <i>Aggregatibacter aphrophilus</i> serum resistance

Abstract

<i>Aggregatibacter actinomycetemcomitans</i> and <i>Aggregatibacter aphrophilus</i> belong to the HACEK group of fastidious Gram-negative organisms, a recognized cause of infective endocarditis. <i>A. actinomycetemcomitans</i> is also implicated in aggressive forms of periodontitis. We demonstrated that <i>A. aphrophilus</i> strains, as <i>A. actinomycetemcomitans</i> are ubiquitously serum resistant. Both species encode two Outer membrane protein A paralogues, here denoted OmpA1 and OmpA2. As their respective pangenomes contain several OmpA1 and OmpA2 alleles, they represent potential genotypic markers. A naturally competent strain of <i>A. actinomycetemcomitans</i> and <i>A. aphrophilus</i>, respectively were used to elucidate if OmpA1 and OmpA2 contribute to serum resistance. Whereas OmpA1 was critical for survival of <i>A. actinomycetemcomitans</i> D7SS in 50% normal human serum (NHS), serum resistant <i>ompA1</i> mutants were fortuitously obtained, expressing enhanced levels of OmpA2. Similarly, OmpA1 rather than OmpA2 was a major contributor to serum resistance of <i>A. aphrophilus</i> HK83. Far-Western blot revealed that OmpA1^AA, OmpA2^AA, and OmpA1^AP can bind to C4-binding protein, an inhibitor of classical and mannose-binding lectin (MBL) complement activation. Indeed, <i>ompA1</i> mutants were susceptible to these pathways, but also to alternative complement activation. This may at least partly reflect a compromised outer membrane integrity but is also consistent with alternative mechanisms involved in OmpA-mediated serum resistance.