The intD mobile genetic element from Dichelobacter nodosus, the causative agent of ovine footrot, is associated with the benign phenotype.

The Gram-negative anaerobic pathogen Dichelobacter nodosus is the principal causative agent of footrot in sheep. The intA, intB and intC elements are mobile genetic elements which integrate into two tRNA genes downstream from csrA (formerly glpA) and pnpA in the D. nodosus chromosome. CsrA homologues act as global repressors of virulence in several bacterial pathogens, as does polynucleotide phosphorylase, the product of pnpA. We have proposed a model in which virulence in D. nodosus is controlled in part by the integration of genetic elements downstream from csrA and pnpA, altering the expression of these putative global regulators of virulence. We describe here a novel integrated genetic element, the intD element, which is 32kb in size and contains an integrase gene, intD, several genes related to genes on other integrated elements of D. nodosus, a type IV secretion system and a putative mobilisation region, suggesting that the intD element has a role in the transfer of other genetic elements. Most of the D. nodosus strains examined which contained the intD gene were benign, with intD integrated next to pnpA, supporting our previous observation that virulent strains of D. nodosus have the intA element next to pnpA.

Deletion of the C-terminus of polynucleotide phosphorylase increases twitching motility, a virulence characteristic of the anaerobic bacterial pathogen Dichelobacter nodosus.

The Gram-negative anaerobe Dichelobacter nodosus is the causative agent of footrot in sheep. Different strains of D. nodosus cause disease of differing severities, ranging from benign to virulent. Virulent strains have greater twitching motility and secrete proteases that are more thermostable than those secreted by benign strains. We have identified polynucleotide phosphorylase (PNPase) as a putative virulence regulator and have proposed that PNPase expression is modulated by the adjacent integration of genetic elements. In this study, we compared PNPase activity in three virulent and four benign strains of D. nodosus and found that PNPase activity is lower in virulent strains. We disrupted the pnpA gene in three benign D. nodosus strains and two virulent strains and showed that deletion of the S1 domain of PNPase reduced catalytic activity. In all but one case, deletion of the PNPase S1 domain had no effect on the thermostability of extracellular proteases. However, this deletion resulted in an increase in twitching motility in benign, but not in virulent strains. Reconstruction of the pnpA gene in two mutant benign strains reduced twitching motility to the parental level. These results support the hypothesis that PNPase is a virulence repressor in benign strains of D. nodosus.