Effects of oligochitosan on the growth, immune responses and gut microbes of tilapia (Oreochromis niloticus)

Abstract

The immunomodulatory effects of oligochitosan have been demonstrated in several fish. However, the underlying mechanisms are not well characterized. The profound interplay between gut microbes and aquaculture has received much scientific attention but understanding the alternations of microbes populating in gut of tilapia (Oreochromis niloticus) fed with oligochitosan remains enigmatic. In this study, the effects of oligochitosan on the growth, immune responses and gut microbes of tilapia were investigated. The feeding trial was conducted in triplicates with the control diet supplemented with oligochitosan at different concentrations (0, 100, 200, 400 or 800 mg/kg). Following a six-week feeding trial, body weights of the fish supplemented with 200 mg/kg and 400 mg/kg oligochitosan were significantly higher than that of the control group. To address the immune responses stimulated by oligochitosan, by the quantitative real time PCR (qRT-PCR), the mRNA expression levels of CSF, IL-1β, IgM, TLR2 and TLR3 genes from head kidney were all significantly up-regulated in the 400 mg/kg group compared to the control. To characterize the gut microbes, bacterial samples were collected from the foregut, midgut, and hindgut, respectively and were subjected to high-throughput sequencing of 16S rDNA. The results showed that significantly lower abundance of Fusobacterium was detected in the hindgut of 400 mg/kg group compared to the control. Additionally, beta-diversity revealed that both gut habitat and oligochitosan had effects on the gut bacterial assembly. To further elucidate the mechanism underlying the effects of oligochitosan on bacterial assembly, the results showed that difference dosages of dietary oligochitosan could alter the specific metabolic pathways and functions of the discriminatory bacterial taxa, resulting in the different bacterial assemblies. To test the antibacterial ability of tilapia fed with oligochitosan, when the tilapias were challenged with Aeromonas hydrophila, the mortality of groups fed with dietary oligochitosan was significantly lower than that of the control. Taken together, appropriate dietary oligochitosan could improve growth, immune responses and alter the bacterial flora in the intestine of tilapia, so as to play a role in fighting against the bacterial infection.

Keywords: Disease resistance; Gut microbes; Immune response; Oligochitosan; Oreochromis niloticus.