Increased abundances of potential pathogenic bacteria and expressions of inflammatory cytokines in the intestine of oyster Crassostrea gigas after high temperature stress.

High temperature stress is a significant threat to the health of oysters, but the effects on their intestinal performances are not well understood. In this study, the effects of high temperature stress on the intestinal histology, immune response and associated microbiota were investigated in Crassostrea gigas after rearing at 20, 25 and 28 °C for 21 days. With the increase of temperature, shortened and shed microvilli as well as increased goblet cells were observed in the intestines of oysters. The transcripts of cytokines CgIL17-5, CgTNF-2 and CgTGF-β and apoptosis-related gene CgCaspase-3 in intestine increased with the increasing temperature. Further, the diversity and composition of the oyster intestinal microbiota changed after high temperature stress. The 16S rRNA gene copy number per ng of DNA in the T25 (5.16 × 10) and T28 (1.63 × 10) groups were higher than that in the control group (8.62 × 10). The Chao 1 index in the T25 (238.00) and T28 (240.17) groups was lower than that in the control group (279.00). The Shannon index decreased progressively with the increasing temperature, with the value in the T28 group (4.44) significantly lower than that in the control group (5.40) (p < 0.05). The abundances of potential pathogenic bacteria such as Acinetobacter, Pseudomonas, Vibrio and Endozoicomonas increased while that of probiotic bacteria Bacillus decreased after high temperature exposure. Functional prediction indicated that the pathways associated with bacterial proliferation were enriched at 25 °C, while those involved in protein synthesis were blocked at 28 °C. Collectively, these results suggested that high temperature stress led to an increased abundances of potential pathogenic bacteria and expressions of inflammatory cytokines in the intestine, which may consequently affect the functional integrity of the intestinal barrier in oysters.