EmaA, a Potential Virulence Determinant of Aggregatibacter actinomycetemcomitans in Endocarditis

Aggregatibacter actinomycetemcomitans is implicated in the etiology of infective endocarditis.  Our earlier work indicated that an antennae-like, oligomeric, extracellular matrix adhesin A (EmaA) mediates the interaction of this bacterium with acid-solublized collagen in vitro.  Collagen, the main component of extracellular matrix (ECM), predominates in the supporting tissue of cardiac valves. Objectives: The purpose of this study was to investigate whether EmaA contributes to the colonization of A. actinomycetemcomitans on traumatized cardiac valves.  Methods: An in vitro tissue model was developed using rabbit cardiac valves.  The resected valves, with or without the removal of the endothelium were incubated with the wild-type and the isogenic emaA^- mutant strains.  The competition index of the mutant versus the wild-type binding was determined by enumerating colony forming units.  The competition assay was further performed in vivo using an endocarditis rabbit model.  Results: There was no difference in binding between the wild-type and the mutant strains, when the endothelium remained intact.  However, the emaA^- mutant was 5-fold less effective than the wild-type strain in colonizing the exposed ECM.  A 10-fold increase in the binding of the wild-type strain to ECM was observed compared with the intact endothelium.  Similar observations were replicated in an in vivo endocarditis rabbit model; the emaA^- mutant was 10-fold less effective in the initial infection of the traumatized aortic valve.  Colocalization studies indicated that A. actinomycetemcomitans bound to Type I collagen.  Conclusion: A. actinomycetemcomitans preferentially colonized the ECM, and together with the evidence that EmaA interacts with the native collagen, suggest that this adhesin is likely a potential virulence determinant of this bacterium in the initiation of infective endocarditis.  This research was supported by National Institutes of Health-National Institute of Dental and Craniofacial Research grants RO1-DE13824 & RO1-DE09760 (KPM), and National Institute of Allergy and Infectious Diseases grants K02AI054908 & R01AI47841 (TK).