The gfc operon is involved in the formation of the O antigen capsule in Aeromonas hydrophila and contributes to virulence in channel catfish

Authors: THURLOW, CHARLES - Auburn University; HOSSAIN, MOHAMMAD - Auburn University; SUN, DAWEI - Auburn University; BARGER, PRISCELLA - Auburn University; FOSHEE, LUKE - Auburn University; Beck, Benjamin; NEWTON, JOSEPH - Auburn University; TERHUNE, JEFFREY - Auburn University; SAPER, MARK - University Of Michigan; LILES, MARK - Auburn University

Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/21/2019
Publication Date: 7/24/2019
Citation: Thurlow, C.M., Hossain, M.J., Sun, D., Barger, P., Foshee, L., Beck, B.H., Newton, J.C., Terhune, J.S., Saper, M.A., Liles, M.R. 2019. The gfc operon is involved in the formation of the O antigen capsule in Aeromonas hydrophila and contributes to virulence in channel catfish. Aquaculture. https://doi.org/10.1016/j.aquaculture.2019.734334.
DOI: https://doi.org/10.1016/j.aquaculture.2019.734334

Interpretive Summary: A highly virulent pathotype of Aeromonas hydrophila has caused epidemic outbreaks of disease in farmed fish in China and the United States and is the leading cause of mortality in the US aquaculture industry. This study describes a structurally unique O antigen capsular polysaccharide present in vAh strains and investigated the significance of the O antigen capsule in vAh virulence. Interestingly, the LPS-associated O antigen was not found to be critical for virulence, whereas the gfc operon required for capsular polysaccharide export and assembly was found to be essential for virulence. A gfcD mutant was attenuated in its virulence to channel catfish and was furthermore found to be disrupted in its ability to form a biofilm, had altered protein secretion, and protected fish from subsequent challenge by wild-type vAh. These studies reveal the importance of the O antigen capsular assembly process in the virulence of this A. hydrophila pathotype.

Technical Abstract: A hypervirulent A. hydrophila (vAh) pathotype has been identified as the etiologic agent responsible for disease outbreaks in farmed carp and catfish species in China and the Southeastern United States, respectively. To assess the role of the LPS O-antigen in vAh virulence, the O-antigen ligase (waaL) and O-antigen polymerase (wzy) genes were deleted. While neither waaL nor wzy were found to be required for vAh virulence, a waaL mutant was found to have a polar effect on an adjacent group 4 capsule (gfc) genetic operon that was predicted to play a role in capsule assembly. Mutations in the gfc operon attenuated vAh virulence in channel catfish, particularly the mutant lacking gfcD that is predicted to encode an outer membrane protein required for capsular polysaccharide export. Furthermore, the vAh gfcD mutant was found to lack significant biofilm-forming capacity or buoyancy compared to wild-type vAh and induced an adaptive immune response that protected catfish from vAh challenge. This study indicates the importance of the capsular polysaccharide assembly process in the pathogenesis of this highly virulent A. hydrophila pathotype.