Impact of oral and waterborne administration of rhamnolipids on the susceptibility of channel catfish (Ictalurus punctatus) to Flavobacterium columnare infection

Authors: ZHANG, DONGDONG - Auburn University; Beck, Benjamin; Lange, Miles; ZHAO, HONGGANG - Auburn University; THONGDA, WILAWAN - Auburn University; YE, ZHI - Auburn University; LI, CHAO - Qingdao Agricultural University; PEATMAN, ERIC - Auburn University

Submitted to: Fish and Shellfish Immunology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/7/2016
Publication Date: 11/3/2016
Citation: Zhang, D., Beck, B.H., Lange, M.D., Zhao, H., Thongda, W., Ye, Z., Li, C., Peatman, E. 2016. Impact of oral and waterborne administration of rhamnolipids on the susceptibility of channel catfish (Ictalurus punctatus) to Flavobacterium columnare infection. Fish and Shellfish Immunology. 60:44-49.

Interpretive Summary: Columnaris disease, caused by the bacterium Flavobacterium columnare, is one of the most problematic diseases of all freshwater fish. Columnaris disease is particularly common and lethal in farmed catfish. Previously, our research team found that the sugar L-rhamnose could protect fish from this disease when added to the water just prior to infecting catfish with a lethal dose of columnaris. While highly effective in protecting fish from columnaris, L-rhamnose is very expensive, underscoring the need for alternative cost-effective sources of rhamnose for disease control. One such alternative may be a group of inexpensive and commercially available compounds termed rhamnolipids (RLs). In this study, we exposed fish to RLs in the feed and water and found that RLs offered no protection against columnaris. Treatment with RLs resulted in changes to a number of immune related factors in the mucosal tissues. While not capable of offering disease resistance, RLs could have potential as adjuvants in priming mucosal tissues to better mount an immune response to vaccination.

Technical Abstract: Flavobacterium columnare is the causative agent of columnaris disease and causes tremendous morbidity and mortality of farmed fish globally. Previously, we identified a potential lectin-mediator (a rhamnose-binding lectin; RBL1a) of F. columnare adhesion and showed higher RBL1a expression in susceptible channel catfish under basal conditions and following infection. Exposure of challenged fish to the carbohydrate ligand L-rhamnose just prior to a challenge substantially decreased columnaris mortality and pathogen adherence via the down-regulation of RBL1a. While highly effective in protecting fish from columnaris, L-rhamnose is prohibitively expensive, underscoring the need for alternative cost-effective sources of rhamnose for disease control. One such alternative may be microbially produced glycolipid compounds termed rhamnolipids (RLs), which feature abundant L-rhamnose moieties and are readily available from commercial sources. In the present study, we examined whether commercially available RLs (administered either by immersion or via feed) would function similarly to L-rhamnose in affording host protection against F. columnare. A four-week feeding trial with basal and RL top-coated diets (basal diet + RLs) was conducted in channel catfish fingerlings. Surprisingly, columnaris challenges revealed significantly lower survival following the 10-day challenge period in RL diet fed fish when compared with the basal treatment group (p < 0.001). In fish fed RLs, we observed a rapid and large-scale upregulation of RBL1a immediately after challenge combined with a suppression of mucin and lysozyme transcripts. Similarly, fish that were briefly pre-exposed to RLs by immersion and then challenged exhibited lower survival as compared to unexposed fish during a 4-day trial. In conclusion, RLs do not represent an alternative to rhamnose as an experimental treatment for protecting catfish from columnaris mortality. Further research is needed to find other affordable and efficacious alternative sources of L-rhamnose.