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Normal intestinal microbiota play an essential role in host health, whereas intestinal dysbiosis is closely associated with a variety of inflammatory diseases. It has been confirmed that the use of antibiotics can induce intestinal dysbiosis in fish. However, it is not clear whether intestinal dysbiosis affects the susceptibility of farmed fish to bacterial infection. In this study, a grass carp model of antibiotic-induced intestinal dysbiosis (AID) was used in challenge experiments to determine the susceptibility to bacterial infection. AID fish were challenged by either immersion or anal intubation with a virulent strain of Aeromonas hydrophila. After immersion challenge, AID fish developed prominent inflammatory symptoms and showed high mortality compared with controls. In immersion-challenged AID fish, the bacterial load in blood and intestinal contents was significantly higher. The immersion challenge also resulted in a significant reduction in the spleen index of AID fish. Histological examination showed severe injury in the intestine, liver, and spleen occurred in immersion-challenged AID fish. Furthermore, AID fish had significantly higher mRNA levels of il1β , il6 and il17a/f1 , and lower levels of il17n , il23r and tnfα in intestinal tissue than control fish. Overall, our results indicate that the susceptibility of grass carp to A. hydrophila infection increased due to intestinal dysbiosis. Therefore, intestinal dysbiosis can be proposed as a risk factor for severe bacterial infection in cultured fish. • Susceptibility to A. hydrophila infection increased in grass carp with intestinal dysbiosis. • Immersion challenge caused high mortality in fish with intestinal dysbiosis. • Immersion and anal intubation evoked different inflammatory responses, regardless of microbiota status. [ABSTRACT FROM AUTHOR]

Normal intestinal microbiota play an essential role in host health, whereas intestinal dysbiosis is closely associated with a variety of inflammatory diseases. It has been confirmed that the use of antibiotics can induce intestinal dysbiosis in fish. However, it is not clear whether intestinal dysbiosis affects the susceptibility of farmed fish to bacterial infection. In this study, a grass carp model of antibiotic-induced intestinal dysbiosis (AID) was used in challenge experiments to determine the susceptibility to bacterial infection. AID fish were challenged by either immersion or anal intubation with a virulent strain of Aeromonas hydrophila. After immersion challenge, AID fish developed prominent inflammatory symptoms and showed high mortality compared with controls. In immersion-challenged AID fish, the bacterial load in blood and intestinal contents was significantly higher. The immersion challenge also resulted in a significant reduction in the spleen index of AID fish. Histological examination showed severe injury in the intestine, liver, and spleen occurred in immersion-challenged AID fish. Furthermore, AID fish had significantly higher mRNA levels of il1β , il6 and il17a/f1 , and lower levels of il17n , il23r and tnfα in intestinal tissue than control fish. Overall, our results indicate that the susceptibility of grass carp to A. hydrophila infection increased due to intestinal dysbiosis. Therefore, intestinal dysbiosis can be proposed as a risk factor for severe bacterial infection in cultured fish. • Susceptibility to A. hydrophila infection increased in grass carp with intestinal dysbiosis. • Immersion challenge caused high mortality in fish with intestinal dysbiosis. • Immersion and anal intubation evoked different inflammatory responses, regardless of microbiota status. [ABSTRACT FROM AUTHOR]