“Fishing for protection: Evaluating recombinant Flavobacterium covae vaccines in channel catfish with or without a dietary probiotic (6916)”

Fishing for protection: Evaluating recombinant Flavobacterium covae vaccines in channel catfish with or without a dietary probiotic

Introduction: Flavobacterium covae, an etiological agent of columnaris disease, is highly virulent in channel catfish and causes significant economic losses for the US aquaculture industry. We tested the immunogenicity of different F. covae proteins, expressed highly under biofilm conditions, to evaluate their efficacy as recombinant subunit vaccines with or without the inclusion of a dietary probiotic. Methods: Recombinant catalase and ferroxidase were administered as either bath immersion or orally via top-coating feed. First, groups of channel catfish (n=540) were immunized by bath immersion with the recombinant protein(s) (1 μg/mL) or sham immunized. Mucosal immunity was evaluated from skin explants via antibody titers and qPCR of immune-related genes. Next, channel catfish fingerlings (n=92) were fed the vaccine or placebo a rate of 3% of body weight for two weeks. Ferroxidase was further evaluated for its vaccine potential with dietary probiotic Bacillus velezensis AP193. Commercial fish feed was coated with B. velezensis AP193 spores at 8% (w/v) for a final concentration of 1×10⁶ CFU/g of feed and fed to some of the control and experimental groups for two weeks. Then, ferroxidase-coated feed (20 μg/g of feed) was fed to a sub portion of experimental groups for eight days. During all three vaccine trials, laboratory challenges against F. covae were performed between six to nine weeks post-immunization to evaluate protection. Results: Each immersion vaccinated group showed significant survival when challenged with F. covae (>30% compared to the control group) at nine weeks post-immunization. No significant mucosal IgM antibody production was generated among the vaccinated groups; however, significant upregulation of innate and adaptive and immune genes was observed in vaccine groups compared to baseline gene expression in the sham immunized group. Orally delivered catalase and ferroxidase fed at the 20 μg/g of feed rate showed significant survival when challenged with F. covae (>13% compared to adjuvant only) at six weeks post-immunization. When ferroxidase was evaluated with the addition of AP193, no increase in fish survival was observed for the vaccine and probiotic combination, relative to vaccine alone. Conclusion: Oral vaccination with recombinant proteins can induce protection, but ongoing research efforts will evaluate the potential for laboratory adaptation of B. velezensis AP193 to achieve greater secondary metabolite production and to provide better efficacy as a disease biocontrol agent. These results lay the groundwork for potential vaccine candidates to be used synergistically with probiotics to protect farmed fish against columnaris disease during the production cycle.

1. CHURCHMAN, EMILY, AUBURN UNIVERSITY, Presenter
2. Lange, Miles, United States Department of Agriculture, Agricultural Research Service, Aquatic Animal Health Research Unit, Author
3. Quiroz, Victoria, Auburn University, Author
4. DeSilva, Ashley, Auburn University, Author
5. Xu, Tingbi, Auburn University, Author
6. Sankappa, Nithin, Oak Ridge Institute for Science and Education (ORISE), ARS Research Participation Program, Author
7. Justice, Megan, United States Department of Agriculture, Agricultural Research Service, Aquatic Animal Health Research Unit, Author
8. Abernathy, Jason, United States Department of Agriculture, Agricultural Research Service, Aquatic Animal Health Research Unit, Author
9. Liles, Mark, Auburn University, Author