The type IX secretion system is required for virulence of the fish pathogen Flavobacterium psychrophilum

Authors: BARBIER, PAUL - University Of Wisconsin; ROCHAT, TATIANA - Institut National De La Recherche Agronomique (INRA); MOHAMMED, HAITHAM - University Of Wisconsin; Wiens, Gregory - Greg; BERNARDET, JEAN-FRANÇOIS - Institut National De La Recherche Agronomique (INRA); HALPERN, DAVID - Institut National De La Recherche Agronomique (INRA); DUCHAUD, ERIC - Institut National De La Recherche Agronomique (INRA); MCBRIDE, MARK - University Of Wisconsin

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/6/2020
Publication Date: 6/12/2020
Citation: Barbier, P., Rochat, T., Mohammed, H.H., Wiens, G.D., Bernardet, J., Halpern, D., Duchaud, E., McBride, M.J. 2020. The type IX secretion system is required for virulence of the fish pathogen Flavobacterium psychrophilum. Applied and Environmental Microbiology. AEM.00799-20. https://doi.org/10.1128/AEM.00799-20.
DOI: https://doi.org/10.1128/AEM.00799-20

Interpretive Summary: Bacterial cold-water disease, caused by F. psychrophilum, is a major problem for salmonid aquaculture. Little is known regarding the virulence factors involved in this disease, and control measures are inadequate. A targeted gene deletion method was adapted to F. psychrophilum and used to demonstrate the importance of the type 9 secretion system in virulence. Proteins secreted by this system are likely virulence factors, and targets for the development of control.

Technical Abstract: Flavobacterium psychrophilum causes bacterial cold-water disease in wild and aquaculture-reared fish, and is a major problem for salmonid aquaculture. The mechanisms responsible for cold-water disease are not known. It was recently demonstrated that the related fish pathogen, Flavobacterium columnare, requires its type IX protein secretion system (T9SS) to cause disease. T9SSs are common in, but apparently confined to the phylum Bacteroidetes, to which members of the genus Flavobacterium belong. T9SSs secrete cell-surface adhesins, gliding motility proteins, peptidases, and other enzymes, any of which may be virulence factors. The F. psychrophilum genome has genes encoding the required components of a T9SS, and is predicted to secrete at least 49 proteins using this system. Here, we used a SacB-mediated gene deletion technique recently adapted for use in the Bacteroidetes, to delete the core F. psychrophilum T9SS gene gldN. The 'gldN mutant cells were deficient for secretion of many proteins in comparison to wild-type cells. Complementation of the mutant with wild-type gldN on a plasmid restored secretion. Compared to wild-type and complemented strains, the 'gldN mutant was deficient in adhesion, gliding motility, and in extracellular proteolytic and hemolytic activities. The 'gldN mutant exhibited reduced virulence in rainbow trout and complementation restored virulence, suggesting that the T9SS plays an important role in the disease.