A Nonfimbrial Adhesin of Aggregatibacter actinomycetemcomitans Mediates Biofilm Biogenesis

Abstract

Periodontitis is an inflammatory disease caused by polymicrobial biofilms. The periodontal pathogen Aggregatibacter actinomycetemcomitans 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.

Aggregatibacter, a Low Abundance Pathobiont That Influences Biogeography, Microbial Dysbiosis, and Host Defense Capabilities in Periodontitis: The History of a Bug, and Localization of Disease

“…In vitro studies indicate that A. actinomycetemcomitans binds in a linear fashion to salivary coated hydroxyapatite, shows minimal specificity, and thus never reaches a saturation point relative to surface binding [23,38]. Further, it has been shown that these fibrils provided a surface for glycoprotein and/or polysaccharide accumulation that also participates in the attachment process ( Figure 3) [27,39].…”

Aggregatibacter Actinomycetemcomitans: Clinical Significance of a Pathobiont Subjected to Ample Changes in Classification and Nomenclature

“…Widespread colonization island or the tad locus [65] Autotransporter adhesins Aae, EmaA and Omp100/ApiA [66][67][68] A leukotoxin of the RTX family with specificity for leukocytes [69] Growth-inhibitory factor cytolethal distending toxin [70,71] CagE, capable of inducing apoptosis [72] Dispersin B, a biofilm-releasing beta-hexosaminidase [73] Vesicle-independent extracellular pelease of proinflammatory lipoprotein [74] The conspicuous growth in broth as small granules adhering to the walls of the test tube was specifically associated with rough colony variants [80]. Mutation analysis of the naturally competent strain D7S revealed flp-1, rcpA, rcpB, tadB, tadD, tadE and tadF to be indispensable for expression of fimbriae, while mutants of five other genes expressed reduced levels of fimbriae, or fimbriae that had different gross appearance [81].…”

Contribution of adhesion proteins to Aggregatibacter actinomycetemcomitans biofilm formation

“…Alternatively, these posttranslational modifications may contribute to the three‐dimensional folding of the adhesin to form the specific structures required for binding (Tang et al., 2012). Both fimbriae and nonfimbrial adhesins target either a single substrate or bind to multiple disparate macromolecules, as well as participating in biofilm formation and invasion of mammalian cells (Danforth et al., 2019; Fine et al., 1999,2005; Mintz, 2004).…”