Actinomyces spp. exhibit type 1 fimbria-mediated adhesion to salivary acidic proline-rich proteins (PRPs) and statherin ligands. Actinomyces spp. with different animal and tissue origins belong to three major adhesion types as relates to ligand specificity and type 1 fimbria genes. (i) In preferential acidic-PRP binding, strains of Actinomyces naeslundii genospecies 1 and 2 from human and monkey mouths displayed at least three ligand specificities characterized by preferential acidic-PRP binding. Slot blot DNA hybridization showed seven highly conserved type 1 fimbria genes (orf1-to -6 and fimP) in genospecies 1 and 2 strains, except that orf5 and orf3 were divergent in genospecies 1. (ii) In preferential statherin binding, oral Actinomyces viscosus strains of rat and hamster origin (and strain 19246 from a human case of actinomycosis) bound statherin preferentially. DNA hybridization and characterization of the type 1 fimbria genes from strain 19246 revealed a homologous gene cluster of four open reading frames (orfA to -C and fimP). Bioinformatics suggested sortase (orfB, orf4, and part of orf5), prepilin peptidase (orfC and orf6), fimbria subunit (fimP), and usher-and autotransporterlike (orfA and orf1 to -3) functions. Those gene regions corresponding to orf3 and orf5 were divergent, those corresponding to orf2, orf1, and fimP were moderately conserved, and those corresponding to orf4 and orf6 were highly conserved. Restriction fragment length polymorphism analyses using a fimP probe separated human and monkey and rat and hamster strains into phylogenetically different groups. (iii) In statherin-specific binding, strains of A. naeslundii genospecies 1 from septic and other human infections displayed a low-avidity binding to statherin. Only the orf4 and orf6 gene regions were highly conserved. Finally, rat saliva devoid of statherin bound bacterial strains avidly irrespective of ligand specificity, and specific antisera detected either type 1, type 2, or both types of fimbria on the investigated Actinomyces strains.Adhesion of commensal and pathogenic bacteria to host tissue surfaces is a crucial event in colonization and infections (13,19). Commensal bacterial species, which protect against pathogens by competing for host binding sites (47), may involve a diversity of adhesion types with multiple ecological niches (42,45).Actinomyces naeslundii and Actinomyces viscosus are dominant commensal Actinomyces spp. colonizing dental and mucosal surfaces of various animal hosts. They show extensive phenotypic and serologic variations (23). Human strains of A. naeslundii were recently grouped into genospecies 1 (A. naeslundii serotype I) and genospecies 2 (A. naeslundii serotypes II, III, and NV and A. viscosus serotype II) based on genetic relatedness (23). A. viscosus serotype I is the dominant species in the rat and hamster mouths. Actinomyces spp. have also been implicated in caries (34), periodontitis (24), and root canal infections (46), as well as in actinomycosis and septic infections (37).The animal and tiss...
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