Mechanistic insights into midgut injury in Bombyx mori induced by low concentration exposure to Bacillus thuringiensis.

Bacillus thuringiensis (Bt) is widely used for the control of agricultural insect pests; however, trace residues of Bt may induce sublethal effects in insects. In this study, Bombyx mori was used as a model insect to investigate the mechanisms underlying midgut injury and tissue apoptosis induced by continuous exposure to a low concentration of Bt. The results showed that prolonged exposure to a low concentration of Bt progressively worsened structural damage to midgut epithelial cells, accompanied by chromatin condensation. Moreover, the apoptotic rate of midgut cells was significantly higher in Bt-exposed silkworms than in the control group. Quantitative transcriptomic analysis indicated that the mitochondria-mediated intrinsic apoptotic pathway was activated earlier at 48 h, whereas activation of the death receptor-mediated extrinsic apoptotic pathway was delayed until 72 h. Consistent with these transcriptional changes, the protein expression level of Caspase-9 showed a sustained increase from 48 h to 96 h, while that of Caspase-8 began to increase at 72 h. Injection of the pan-caspase inhibitor Z-VAD-FMK into Bt-treated silkworms significantly suppressed Caspase-3 protein expression and its enzymatic activity, and markedly alleviated Bt-induced abnormal developmental phenotypes. These findings provide a basis for evaluating the biosafety risks associated with low-concentration environmental residues.