Coaggregation Between Actinomyces naeslundii and Streptococcus oralis

The gram-positive oral bacterium Actinomyces naeslundii T14V possesses two distinct types of fimbriae (types 1 and 2). Type 1 fimbriae mediate the attachment of this organism to the salivary pellicle of teeth, while Type 2 fimbriae mediate lactose-sensitive coaggregations with certain streptococci and adhesion to host cells. Available DNA sequences ( Hoflack & Yeung, 2001) indicate that the locus for type 2 fimbria production includes three genes, fimB for a putative protein with unknown function, fimA for the major structural subunit, and srtC2 for a type 2 fimbria-specific sortase involved in the assembly of the fimbriae. Objective: To determine the role of fimB in the surface expression of A. naeslundii type 2 fimbriae. Methods: Wild type A. naeslundii was transformed with the DNA construct containing partially deleted fimB. A fimB deletion mutant was obtained by allelic exchange and confirmed by PCR and junction sequencing. Western blotting with a monoclonal antibody against FimA was used to assess the assembly of type 2 fimbriae by this mutant. Wild type and mutant strains were also compared for their coaggregations with Streptococcus oralis 34, an established coaggregation partner for A. naselundii. Results: The isolated fimB deletion mutant was able to assemble type 2 fimbriae on the cell surface, but the patterns were different from that of the wild type. Compared to the wild type, the level of coaggragation between this mutant and S. oralis 34 was significantly reduced. Conclusion: These results suggest that FimB is involved in A. naeslundii type 2 fimbria assembly and plays a critical role in mediating the coaggregation between A. naeslundii and S. oralis 34.