Objectives: Cells of the periodontal pathogen Actinobacillus actinomycetemcomitans exhibit tight adherence during growth, a property that has been shown to be essential for virulence in a rat model. Tight adherence is mediated by adhesive fimbriae that form on the surface of the cell. We sought to determine the role of exopolysaccharide in the adherence phenotype exhibited by A. actinomycetemcomitans. Methods: We identified a gene in the A. actinomycetemcomitans unfinished genome sequence, designated exoA which encodes a protein homologous to exopolysaccharide biosynthetic genes in other bacteria. We constructed an isogenic mutant of A. actinomycetemcomitans strain IDH781 that carried a transposon insertion in exoA, along with a mutant strain that contained an insertion in flp-1, which encodes the major protein subunit of the adhesive fimbriae. We tested the wild-type and mutant strains in a colorimetric plastic binding assay and in a visual auto-aggregation assay. Results: The flp-1 mutant strain exhibited significantly reduced binding to polystyrene. A₅₉₀ values dropped from 3.25 for wild type stain bind to 0.35 for the flp-1 mutant strain. This represents a greater than 100 fold decrease in cell binding. The exoA mutant strain exhibited wild-type levels of plastic binding. Cells of both mutant strains failed to auto-aggregate in solution. When cultured on agar, the flp-1 mutant produced smooth colonies whereas the exoA mutant produced rough colonies, which lacked the internal, star-shaped structure characteristic of fresh A. actinomycetemcomitans clinical isolates. Conclusion: The gene exoA plays a role in A. actinomycetemcomitans auto-aggregation but not in surface adherence.

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