Different type 1 fimbrial genes and tropisms of commensal and potentially pathogenic Actinomyces spp. with different salivary acidic proline-rich protein and statherin ligand specificities

Actinomyces spp. exhibit type 1 fimbria-mediated adhesion to salivary acidi c proline-rich proteins (PRPs) and statherin ligands. Actinomyces spp. with different animal and tissue origins belong to three major adhesion types a s relates to ligand specificity and type 1 fimbria genes. (i) In preferenti al acidic-PRP binding, strains of Actinomyces naeslundii genospecies 1 and 2 from human and monkey mouths displayed at least three ligand specificitie s characterized by preferential acidic-PRP binding. Slot blot DNA hybridiza tion showed seven highly conserved type 1 fimbria genes (orf1- to -6 and fi mP) in genospecies 1 and 2 strains, except that orf5 and orf3 were divergen t in genospecies 1. (ii) In preferential statherin binding, oral Actinomyce s viscosus strains of rat and hamster origin (and strain 19246 from a human case of actinomycosis) bound statherin preferentially. DNA hybridization a nd 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 autotran sporter-like (orfA and orf1 to -3) functions. Those gene regions correspond ing to orf3 and orf5 were divergent, those corresponding to orf2, orf1, and fimP were moderately conserved, and those corresponding to orf4 and orf6 w ere highly conserved. Restriction fragment length polymorphism analyses usi ng a fimP probe separated human and monkey and rat and hamster strains into phylogenetically different groups. (iii) In statherin-specific binding, st rains 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 boun d bacterial strains avidly irrespective of ligand specificity, and specific antisera detected either type 1, type 2, or both types of fimbria on the i nvestigated Actinomyces strains.