Abstract: 【Aim】 Gut microbiota might play an important role in mediating host resistance to Bacillus thuringiensis (Bt). The purpose of this study is to explore the effect of gut bacteria on the insecticidal activity of Bt in the diamondback moth, Plutella xylostella, and analyze the action mechanism of gut bacteria in host protection. 【Methods】 The survival rates of the 3rd instar larvae of P. xylostella feeding on the sterile artificial diet and on the artificial diet containing total gut bacteria, Enterobacter sp. IAE5 (EbPXG5), Bt strain Bt8010, Bt8010+total gut bacteria and Bt8010+EbPXG5, respectively, as well as the survival rates of P. xylostella after feeding on the sterile artificial diet and artificial diet containing EbPXG5 supernatant, EbPXG5 cell lysate solution, Bt8010+EbPXG5 supernatant, Bt8010+EbPXG5 cell lysate solution and Bt8010, respectively, were measured, and then the influence of gut bacteria on the Bt susceptibility of P. xylostella at different time was analyzed were analyzed. The plate culture technique was used to assay the abundance of EbPXG5 and Bt8010 in the gut and haemolymph of the 3rd instar larvae of P. xylostella fed with the artificial diets containing EbPXG5, Bt8010 and Bt8010+EbPXG5, respectively, and the inhibitory effect of EbPXG5 on Bt8010 in vitro, and the effect of gut bacteria on the proliferation of Bt8010 in the gut of P. xylostella and their invasion into the blood cavity was analyzed. The inner wall morphology of the gut tissues of the sterile 3rd instar larvae of P. xylostella and the gut tissues of P. xylostella fed with EbPXG5, Bt8010 and EbPXG5+Bt8010, respectively, were observed by scanning electron microscope to reveal the protective function of gut bacteria on the inner wall of gut. 【Results】 The survival rates of the 3rd instar larvae of P. xylostella feeding on the diet containing total gut bacteria and the diet containing EbPXG5 had no significant difference compared with that in the control feeding on the sterile artificial diet, but the survival rates of the 3rd instar larvae feeding on the diet containing Bt8010+EbPXG5 and the diet containing Bt8010+total gut bacteria were significantly higher than those feeding on the diet containing Bt8010 at 24, 36, 48 and 60 h. The survival rates of the 3rd instar larvae feeding on the diets containing EbPXG5 supernatant and EbPXG5 cell lysate solution had no difference from that of the control group, and the survival rates of the 3rd instar larvae feeding on the diets containing Bt8010+EbPXG5 supernatant and Bt8010+EbPXG5 cell lysate solution also had no difference from that of the larvae feeding on the diet containing Bt8010 alone. There was no significant difference in the abundance of EbPXG5 in the gut of the 3rd instar larvae of P. xylostella feeding on the artificial diets containing EbPXG5 alone and Bt8010+EbPXG5, respectively, but the abundance of Bt8010 in the gut of the 3rd instar larvae feeding on the diet containing Bt8010+EbPXG5 was significantly lower than that feeding on the diet containing Bt8010 alone at 24, 36 and 48 h. The abundance of EbPXG5 in the haemolymph of P. xylostella fed with the diet containing Bt8010+EbPXG5 was higher than that fed with the diet containing EbPXG5 alone at 36 and 48 h; however, the abundance of Bt8010 in the haemolymph of P. xylostella fed with the diet containing Bt8010+EbPXG5 was significantly lower than that fed with the diet contaning Bt8010 alone at 36 and 48 h. The Oxford cup bacteriostatic test showed that EbPXG5 had no inhibitory effect on Bt8010 in vitro. The results of scanning electron microscope observation showed that Bt8010 could destroy the larval gut cavity to form holes, mediate Bt8010 and other bacteria to cross the gut barrier and enter the hemolymph of P. xylostella. EbPXG5 could colonize the inner wall of the gut cavity of P. xylostella, weaken the damage of Bt8010 to the inner wall of the gut, and reduce the abundance of Bt8010 in the gut and hemolymph. 【Conclusion】 Gut bacteria EbPXG5 plays an important role in protecting P. xylostella and reducing the susceptibility of P. xylostella to Bt. It may weaken the colonization and invasion of pathogens by competing for niche and protecting the inner wall of gut, thus reducing the susceptibility of host to Bt. The results have important reference value for promoting the biological control and integrated management of P. xylostella.

Key words: Plutella xylostella, gut microbiota; Bacillus thuringiensis; Enterobacter sp., colonization resistance, Bt sensitivity