CHARACTERIZATION OF FLAA(-) AND FLAB(-) MUTANTS OF SERPULINA-HYODYSENTERIAE - BOTH FLAGELLIN SUBUNITS, FLAA AND FLAB, ARE NECESSARY FOR FULL MOTILITY AND INTESTINAL COLONIZATION

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 allel es (flaA1 and flaB1) were constructed and examined for virulence and a bility to colonize the intestinal tract of mice, Mice challenged with the wild-type, parent strain showed a dose-related response to the cha llenge organism. In contrast, all flagellar mutant strains demonstrate d aberrant motility in vitro and a significantly reduced ability to co lonize and infect mice. To some extent, this degree of reduction in co lonizing ability was dependent on the wild-type background strain used for mutant construction. A flaB1(-) strain generated from a 'laborato ry isolate' was unable to colonize the mouse gut even at high challeng e doses, although its parent was virulent for mice. However, when the same parent strain was 'animal-passed' prior to disruption of flaB1, t he resulting flaB1(-) strain was able to transiently colonize the mous e gut and induce intestinal lesions, A comparison of a series of flage llar mutants constructed using the animal-passed parent strain further revealed that specific inactivation of flaB1 resulted in a more prono unced reduction in virulence and colonizing ability than that which oc curred with two flaA1 mutants. Taken together, these data suggest that motility is an essential virulence factor of S. hyodysenteriae and th at both sheath and core flagellin subunits, FlaA and FlaB, are necessa ry for full motility and intestinal colonization.