Transformation-mediated serogroup conversion of Dichelobacter nodosus.

Dichelobacter nodosus is the essential causative agent of footrot in sheep. The type IV fimbriae of D. nodosus are required for virulence, are highly immunogenic and immunoprotective, and can be divided into 10 major serogroups. Fimbrial variation has been postulated to have arisen because of genetic recombination within the fimbrial gene region perhaps as a means of evading the immune response invoked by infection. To show that antigenic variation in these fimbriae could occur after natural transformation and subsequent homologous recombination, a suicide plasmid containing the fimbrial subunit gene, fimA, of a serogroup G strain was used to convert a serogroup I strain to serogroup G. The resultant mutants were shown by Western blotting and slide agglutination to produce serogroup G fimbriae, but by two independent methods to still have the genotype of the parent type I strain. These data have significant implications for the use of fimbrial vaccines for the control of ovine footrot and suggest that benign strains of D. nodosus could play an important role as a reservoir of alternative fimbrial antigens.