BackgroundAlthough high levels of fish meal replacement by alternative protein sources have been achieved without relevant alterations in terms of growth performance, negative effects on immune status were detected. The diet and fish immunity interactions at gut level have been widely discussed, although in vivo approaches have reported several limitations. In this sense, intestine explant culture system can be a valuable complementary tool to study the interactions between pathogenic bacteria and fish gut response, and the possible influence of environmental, breeding, rearing conditions or dietary components on the responsiveness of the innate immune system in fish.The object of this study was to test the impact of total substitution of fish meal by plant protein on the intestinal health of seabream (12g) in two growth stages: phase I (90 days), up to 68 g, and phase II (305 days), up to 250g. In phase II, the effects of the long term and short exposure (15 days) to plant protein diet were determined. In order to determine the effect of plant protein feeding on the innate immune response to bacterial pathogens, and an ex vivo procedure of intestine explants culture was implemented.ResultsFish showed less tolerance to dietary plant protein in phase I than in phase II, while the ex vivo assays indicated that the intestine from fish fed at short-term plant diets showed a higher immune response than at long term feeding.In relation to the immune response to bacterial challenge, a significant expression in pro-inflammatory cytokines IL-1β and IL-6 after 6 hours of exposure to V. algynoliticus, while COX-2 expression was significantly induced by P. damselae subsp. pisicida, showing positive high correlation between them.ConclusionsA differential health status was observed depending of growth stage, being stricter to the plant protein inclusion the younger fish. The new experimental system based on fish intestinal explants culture has been successfully implemented, becoming an effective methodology for ex vivo studies. Under ex vivo conditions, the bacterial challenge induced inflammatory and immune intestinal response, responding stronger those intestine of fish fed during a short-term with a total substitution of fish meal.