Characterising variation in the gene for the Dichelobacter nodosus Type IV fimbrial protein: Development of a molecular typing system

Abstract

Dichelobacter nodosus is the causative agent of ovine footrot. The Type IV multi-subunit fimbriae of D.nodosus represents the key immunogenic component of footrot vaccines (Stewart 1989). Variation within the gene (jimA) for the fimbrial protein suQ.unit is currently the basis for the serotype classification of these bacteria. To date, no sequencing of the fimA gene has been conducted for New Zealand isolates. This study describes the development of a PCR specific for the variable region of the fimA gene. Cloning and sequencing of a single field derived amplimer and eight cultures currently being used in commercial footrot vaccines (Mallinckrodt Veterinary Ltd), is described. The rimA sequences exhibited varying degrees (89-100%) of homology to the prototype Australian strains (Mattick et ai., 1991). Serotype B sequences of New Zealand origin showed the greatest variation at the protein level. Two of these sequences predicted 11 and 13 amino acid substitutions (compared to the prototype B 1 isolate) in the hypervariable region of the protein between cysteine residues, that is thought to comprise the B-cell epitope. Such sequence variation may account for the ineffectiveness of vaccines which currently employ a majority of isolates of Australian origin. These findings indicate that further characterisation of Jim A gene heterogeneity is required within New Zealand to allow optimisation of footrot vaccines. AfimA based PCR-RFLP typing system was described and potentially represents a quick and cost effective means of typing D.nodosus from lesion material. The development of molecular techniques to rapidly "type" D.nodosus may enable strain-specific vaccines to be formulated, thereby circumventing the problems associated with antigenic competition in polyvalent formulations.