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Journal of Bacteriology, May 2008, p. 3323-3335, Vol. 190, No. 9
0021-9193/08/$08.00+0     doi:10.1128/JB.01807-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Twitching Motility Is Essential for Virulence in Dichelobacter nodosus ^,

Australian Research Council Centre of Excellence in Structure and Functional Microbial Genomics,¹ Department of Microbiology, Monash University, Victoria 3800, Australia,² Elizabeth Macarthur Agricultural Institute, NSW Department of Primary Industries, PMB 8, Camden, New South Wales 2570, Australia,³ Faculty of Veterinary Science, University of Sydney, PMB 3, Camden, New South Wales 2570, Australia⁴

Received 15 November 2007/ Accepted 20 February 2008

Type IV fimbriae are essential virulence factors of Dichelobacter nodosus, the principal causative agent of ovine foot rot. The fimA fimbrial subunit gene is required for virulence, but fimA mutants exhibit several phenotypic changes and it is not certain if the effects on virulence result from the loss of type IV fimbria-mediated twitching motility, cell adherence, or reduced protease secretion. We showed that mutation of either the pilT or pilU gene eliminated the ability to carry out twitching motility. However, the pilT mutants displayed decreased adhesion to epithelial cells and reduced protease secretion, whereas the pilU mutants had wild-type levels of extracellular protease secretion and adherence. These data provided evidence that PilT is required for the type IV fimbria-dependent protease secretion pathway in D. nodosus. It was postulated that sufficient fimbrial retraction must occur in the pilU mutants to allow protease secretion, but not twitching motility, to take place. Although no cell movement was detected in a pilU mutant of D. nodosus, aberrant motion was detected in an equivalent mutant of Pseudomonas aeruginosa. These observations explain how in D. nodosus protease secretion can occur in a pilU mutant but not in a pilT mutant. In addition, virulence studies with sheep showed that both the pilT and pilU mutants were avirulent, providing evidence that mutation of the type IV fimbrial system affects virulence by eliminating twitching motility, not by altering cell adherence or protease secretion.

Published ahead of print on 29 February 2008.

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