EFFECTS OF DIRECT-FED PROBIOTICS AND DISEASE-INDUCED STRESS ON THE METABOLOME OF FISH

Probiotics have been gaining attention by the aquaculture community in recent years because they can be used to improve the overall health and production of fish. More specifically, research has demonstrated that probiotic fed fish are often able to more efficiently utilize nutrients in diets and enhance disease resistance when exposed to pathogens. Even though these enhancements are observed, very little is understood on how or if probiotics initiate biochemical changes at the cellular or systemic level. Fortunately, new laboratory-based tools are now available to empower researchers and the industry to answer these kinds of questions. One-hundred and ninety-two iridescent sharks (Pangasius hypophthalmus) were used for this study. Half of the fish (n=96 were fed a control diet and the other half (n=96) were fed a probiotic diet (Bacillus subtilis). Fish were cultured on these diets for over 8 weeks in independent but identically designed recirculating aquaculture systems with similar water quality (P > 0.05) for dissolved oxygen, ammonia, nitrite, nitrate, pH, and alkalinity. After 8 weeks, half of each group of fish (n=48) per diet were bath exposed to Edwardsiella ictaluri at a sub-lethal level where gross clinical signs of infection occurred.  Table 1 describes the sampling plan.

Pooled samples of plasma and livers were quickly frozen and stored at -80 C until analyzed. All samples were then evaluated using a Waters ACQUITY ultra-performance liquid chromatography (UPLC) and a Thermo Scientific Q-Exactive high resolution/accurate mass spectrometer interfaced with a heated electrospray ionization (HESI-II) source and Orbitrap mass analyzer operated at 35,000 mass resolution. The present data set looked at 530 and 502 named biochemicals in the liver and plasma, respectively. Statistical differences (P<0.05), and approaching significant differences (0.05<P<0.10), were observed for biochemicals between control versus probiotic fish that were either not exposed or exposed to E. ictaluri. Among the differences observed were changes in bile profiles, fatty acid concentrations, antioxidant levels, and shifts in other metabolites such as amino-acid catabolites and endocannabinoids in the plasma.