The fimbrial subunit gene fimA is essential for virulence

D. nodosus possesses fine, filamentous appendages called fimbriae or pili. Fimbraie are one of the key virulence factors of D. nodosus. Fimbriae are composed of a repeating subunit protein, which is encoded by the fimA gene. To examine the role of fimbriae in virulence, a mutant of virulent D. nodosus reference strain VCS1703A was created with an inactivated fimA gene. Analysis of the mutant strain revealed that it no longer produced fimbriae. Furthermore, the mutants were shown to be avirulent in a sheep pen trial.

Kennan, R. M., Dhungyel, O. P., Whittington, R. J., Egerton, J. R., Rood, J. I. 2001, ‘The Type IV Fimbrial Subunit Gene (fimA) of Dichelobacter nodosus Is Essential for Virulence, Protease Secretion, and Natural Competence’, Journal of Bacteriology 183(15), pp. 4451-4458


Dichelobacter nodosus is the essential causative agent of footrot in sheep. The major D. nodosus-encoded virulence factors that have been implicated in the disease are type IV fimbriae and extracellular proteases. To examine the role of the fimbriae in virulence, allelic exchange was used to insertionally inactivate the fimA gene, which encodes the fimbrial subunit protein, from the virulent type G D. nodosus strain VCS1703A. Detailed analysis of two independently derived fimA mutants revealed that they no longer produced the fimbrial subunit protein or intact fimbriae and did not exhibit twitching motility. In addition, these mutants were no longer capable of undergoing natural transformation and did not secrete wild-type levels of extracellular proteases. These effects were not due to polar effects on the downstream fimB gene because insertionally inactivated fimB mutants were not defective in any of these phenotypic tests. Virulence testing of the mutants in a sheep pen trial conducted under controlled environmental conditions showed that the fimA mutants were avirulent, providing evidence that the fimA gene is an essential D. nodosus virulence gene. These studies represent the first time that molecular genetics has been used to determine the role of virulence genes in this slow growing anaerobic bacterium.

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