The type IV fimbrial subunit gene (fimA) of Dichelobacter nodosus is essential for virulence, protease secretion, and natural competence

Dichelobacter nodosus is the essential causative agent of footrot in sheep. The major D. nodosus-encoded virulence factors that have been implicated i n the disease are type IV fimbriae and extracellular proteases. To examine the role of the fimbriae in virulence, allelic exchange was used to inserti onally inactivate the fimA gene, which encodes the fimbrial subunit protein , from the virulent type GD. nodosus strain VCS1703A. Detailed analysis of two independently derived fimA mutants revealed that they no longer produce d the fimbrial subunit protein or intact fimbriae and did not exhibit twitc hing motility. In addition, these mutants were no longer capable of undergo ing natural transformation and did not secrete wild-type levels of extracel lular proteases. These effects were not due to polar effects on the downstr eam fimB gene because insertionally inactivated fimB mutants were not defec tive in any of these phenotypic tests. Virulence testing of the mutants in a sheep pen trial conducted under controlled environmental conditions showe d that the fimA mutants were avirulent, providing evidence that the fimA ge ne is an essential D. nodosus virulence gene. These studies represent the f irst time that molecular genetics has been used to determine the role of vi rulence genes in this slow growing anaerobic bacterium.