Atlantic salmon (Salmo salar ) fed on a carbohydrate-rich diets exhibit suboptimal growth performance along with other metabolic disturbances. It is well known that gut microbes play a pivotal role in influencing metabolism of the host as well as they can be modified by diet. The main goal of the study was to determine the effect of feeding grading levels of digestible carbohydrates to Atlantic salmon on distal intestine digesta microbiota at 3 sampling time (i.e., week 4, week 8 and week 12), during a 12-week trial.
A low carbohydrate-to-high protein diet (LC/HP, 0% wheat starch), a medium carbohydrate-to-medium protein diet (MC/MP, 15% wheat starch) or a high carbohydrate-to-low protein diet (HC/LP, 30% wheat starch) was fed to triplicate fish tanks (27-28 fish per tank). We performed an in-depth characterization of the distal intestine digesta microbiota. Further, growth parameters, liver histology and the expression of genes involved in hepatic neolipogenesis in fish were measured.
Major results are f ish fed on a HC/LP diet showed greater hepatosomatic and viscerosomatic indexes (P = 0.026 and P = 0.018, respectively), lower final weight (P = 0.005), weight gain (P = 0.003), feed efficiency (P = 0.033) and growth rate (P = 0.003) compared with fish fed the LC/HP diet. Further, feeding salmon on a high digestible carbohydrate diet caused greater lipid vacuolization , steatosis index (P = 0.007) and expression of fatty acid synthase (fas ) and delta-6 fatty acyl desaturase (d6fad) (P = 0.001 and P = 0.001, respectively) in the liver compared with fish fed the LC/HP diet. Although, t he major impact of feeding a carbohydrate-rich diet to Atlantic salmon in beta diversity of distal intestine digesta microbiota was observed at week 4 (HC/LP vs MC/MP and HC/LP vs LC/HP ; P = 0.007 and P = 0.008, respectively) and week 8 (HC/LP vs MC/MP; P = 0.04 ), no differences between experimental groups were detected after 12 weeks of feeding. Finally, at the end of the trial, there was a significant negative correlation between lactic acid bacteria members , including Leuconostoc and Lactobacillus, with hepatic steatosis, expression of fas and d6fad and the hepatosomatic and viscerosomatic indexes , and between Weissella and hepatic steatosis and the hepatosomatic index. Further research to explore the potential use of LAB as probiotics to improve liver health in carnivorous fish fed fatty liver-induced diet is warranted.