Characterization of flaA− and flaB− mutants of Serpulina hyodysenteriae: both flagellin subunits, FlaA and FlaB, are necessary for full motility and intestinal colonization

Abstract

Motility of Serpulina hyodysenteriae is thought to play a pivotal role in the enteropathogenicity of this spirochete. To test this, a series of isogenic mutants containing specifically disrupted flagellar alleles ( flaA1 and flaB1) were constructed and examined for virulence and ability to colonize the intestinal tract of mice. Mice challenged with the wild-type, parent strain showed a dose-related response to the challenge organism. In contrast, all flagellar mutant strains demonstrated aberrant motility in vitro and a significantly reduced ability to colonize and infect mice. To some extent, this degree of reduction in colonizing ability was dependent on the wild-type background strain used for mutant construction. A flaB1 − strain generated from a `laboratory isolate' was unable to colonize the mouse gut even at high challenge doses, although its parent was virulent for mice. However, when the same parent strain was `animal-passed' prior to disruption of flaB1, the resulting flaB1 − strain was able to transiently colonize the mouse gut and induce intestinal lesions. A comparison of a series of flagellar mutants constructed using the animal-passed parent strain further revealed that specific inactivation of flaB1 resulted in a more pronounced reduction in virulence and colonizing ability than that which occurred with two flaA1 mutants. Taken together, these data suggest that motility is an essential virulence factor of S. hyodysenteriae and that both sheath and core flagellin subunits, FlaA and FlaB, are necessary for full motility and intestinal colonization.