A Nonfimbrial Adhesin of Aggregatibacter actinomycetemcomitans Mediates Biofilm Biogenesis

Periodontitis is an inflammatory disease caused by polymicrobial biofilms. The periodontal pathogen Aggregatibacter actinornyceterncomitans displays two proteinaceous surface structures, the fimbriae and the nonfimbrial extracellular matrix binding protein A (EmaA), as observed by electron microscopy. Fimbriae participate in biofilm biogenesis and the EmaA adhesins mediate collagen binding. However, in the absence of fimbriae, A. actinomycetemcomitans still retains the potential to form robust biofilms, suggesting that other surface macromolecules participate in biofilm development. Here, isogenic mutant strains lacking EmaA structures, but still expressing fimbriae, were observed to have reduced biofilm potential. In strains lacking both EmaA and fimbriae, biofilm mass was reduced by 80%. EmaA enhanced biofilm formation in different strains, independent of the fimbriation state or serotype. Confocal microscopy revealed differences in cell density within microcolonies between the EmaA positive and mutant strains. EmaA-mediated biofilm formation was found to be independent of the glycosylation state and the precise three-dimensional conformation of the protein, and thus this function is uncorrelated with collagen binding activity. The data suggest that EmaA is a multifunctional adhesin that utilizes different mechanisms to enhance bacterial binding to collagen and to enhance biofilm formation, both of which are important for A. actinomycetemcomitans colonization and subsequent infection.