The flagellar master operon flhDC is a pleiotropic regulator involved in motility and virulence of the fish pathogen Yersinia ruckeri

Authors: Snyder, Anna; GRAF, JOERG - University Of Connecticut; Welch, Timothy - Tim

Submitted to: Journal of Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/26/2016
Publication Date: 12/15/2016
Citation: Snyder, A.K., Graf, J., Welch, T.J. 2016. The flagellar master operon flhDC is a pleiotropic regulator involved in motility and virulence of the fish pathogen Yersinia ruckeri. Journal of Applied Microbiology. 122(3):578-588. doi: 10.1111/jam.13374.

Interpretive Summary: Yersinia ruckeri is the causative agent of enteric red mouth disease (ERM), a bacterial disease that primarily affects farmed rainbow trout. Y. ruckeri-caused disease has been successfully controlled by immersion vaccination for over three decades: however, atypical strains of Y. ruckeri have recently been isolated from disease outbreaks in previously vaccinated fish in the Southeastern United States and throughout Europe. The bacterial isolates responsible for these outbreaks contain genetic mutations causing a loss of flagellar motility and flagellar secretion and have been classified as Y. ruckeri biotype 2 (BT2). In this paper, we have identified the flhD gene as a master regulator controlling flagellar motility in Y. ruckeri. This gene also regulates other diverse genes, consistent with a role for flhD as a pleiotropic regulator. Mutation of the flhD gene confers a competitive advantage during experimental challenge of rainbow trout while the same strain containing a natural BT2-causing mutation does not confer this advantage. This work enhances our knowledge regarding the genetic basis of flagellar regulation and its effect on virulence and thus provides a better understanding of the selective pressures driving emergence of BT2 strains. This will aid in the development of improved vaccines or other management practices aimed at controlling this disease.

Technical Abstract: Aims: To investigate the function of the master flagellar operon flhDC in the fish pathogen Yersinia ruckeri and compare the effect of flhD mutation to a naturally occurring mutation causing loss-of-motility in emergent biotype 2 (BT2) strains. Methods and Results: In this study isogenic Y. ruckeri flhD and fliR mutants were constructed in a motile strain. Both mutations caused loss-of-motility, ablation of flagellin synthesis and phospholipase secretion similar to naturally occurring BT2 strains. Transcriptome analysis confirmed flhDC regulation of flagellar motility, chemotaxis and phospholipase loci as well as other genes of diverse function. Competition experiments indicated that the flhD' mutation confers a competitive advantage within the fish host when compared to its isogeneic parent, while this advantage was not seen with the naturally occurring fliR mutation. Conclusions: Our results demonstrate that flhD is necessary for expression of the flagellar secretion system as well as other diverse loci, consistent with a role for flhD as a pleiotropic regulator. We also show that mutation of flhD confers a competitive advantage during experimental challenge of rainbow trout while an isogenic strain containing the fliR' mutant allele does not confer this advantage. Significance and Impact of Study: Y. ruckeri is the causative agent of enteric red mouth disease, an invasive septicemia that affects farmed salmonid fish species. Disease outbreaks can cause severe economic losses in aquaculture. BT2 variants, which have independently emerged worldwide, are an increasing threat to farmed fish production. Knowledge of mechanisms involved in virulence and conserved functional capabilities among strains may be exploited for the development of novel disease control strategies.