Flagellar regulation mediated by the Rcs pathway is required for virulence in the fish pathogen Yersinia ruckeri

Authors: JOZWICK, ANNA - Goucher College; LA PATRA, SCOTT - Clear Springs Foods, Inc; GRAF, JOERG - University Of Connecticut; Welch, Timothy - Tim

Submitted to: Fish and Shellfish Immunology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/17/2019
Publication Date: 8/1/2019
Citation: Jozwick, A., La Patra, S., Graf, J., Welch, T.J. 2019. Flagellar regulation mediated by the Rcs pathway is required for virulence in the fish pathogen Yersinia ruckeri. Fish and Shellfish Immunology. 91:306.314. https://doi.org/10.1016/j.fsi.2019.05.036.
DOI: https://doi.org/10.1016/j.fsi.2019.05.036

Interpretive Summary: Yersinia ruckeri is the causative agent of enteric red mouth disease (ERM), a bacterial disease that primarily affects farmed rainbow trout. Bacteria adapt to changes in their environment by sensing these changes and responding by modulating gene expression appropriate for the new conditions. In this paper we show that Y. ruckeri has the ability to sense its fish host and responds by turning off genes necessary for production of the bacterial flagellum. We demonstrate that this regulation is dependent on the rcsB regulatory gene and that this regulation is critical for the ability of this pathogen to cause disease. The flagellum is a potent immune stimulant and we speculate that repression of this system during infection is critical for the avoidance of host immune stimulation and thus immune recognition. 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 factors leading to infection. This will aid in the development of improved vaccines or other management practices aimed at controlling this disease.

Technical Abstract: The flagellum is a complex surface structure necessary for a number of activities including motility, chemotaxis, biofilm formation and host attachment. Flagellin, the primary structural protein making up this structure, is an abundant and potent activator of innate and adaptive immunity and therefore expression of flagellin during infection could be deleterious to the infection process due to flagellin-mediated host recognition. Herein, we demonstrate that the flagellin gene (fliC) is repressed during infection and subsequently de-repressed upon host death when tested in rainbow trout. These results suggests that Y. ruckeri possesses a regulatory system capable of sensing live host and modulating the expression of motility in response. Examination of the master flagellar operon (flhDC) promoter region revealed potential interaction with the Rcs pathway through an Rcs(A)B Box. Deletion of rcsB (rcsB') resulted in overproduction of flagellin and unregulated motility, showing that the Rcs pathway negatively regulates biosynthesis of the flagellar apparatus. Experimental challenge with rcsB and rcsBfliC mutants revealed that mutation of the Rcs pathway results in virulence attenuation which is dependent on presence of the flagellin gene. These results suggest that the inappropriate expression of flagellin during infection triggers host recognition and thus immune stimulation resulting in attenuation of virulence. In addition, RNAseq analyses of the rcsB' mutant strain verified the role of this gene as a negative regulator of the flagellar motility system and identified several additional genes regulated by the Rcs pathway.