Damage and recovery of artificial diet rearing silkworms (Bombyx mori L.) exposed to high-concentration florfenicol as well as the dynamic changes of midgut flora after functional bacterium supplementation

Florfenicol (FF) is widely used in animal husbandry due to its broad-spectrum bactericidal activity, while there is little research focused on its toxic effects on the silkworm(Bombyx mori L.), a model organism. It was demonstrated in this paper that high-concentration florfenicol exposure significantly inhibited the activities of digestive and antioxidant enzymes, reduced the body weight and cocooning ability of silkworms, prolonged the instar duration, and simultaneously led to organelle swelling and vacuolization in the midgut, a large number of autophagosomes, and nuclear condensation. Meanwhile, it was found that exposure to FF reduced the α diversity and richness of the midgut flora, leading to a severe loss of core microbiota. The results of this study also indicated that Bacillus velezensis LY5, as a functional bacterium, effectively mitigated the toxic effects of FF on silkworm larvae. This strain significantly improved the body weight and cocoon quality of silkworms exposed to FF, promoted the repair of midgut tissue damage, and restored the structure of the midgut core flora. Furthermore, B. velezensis LY5 treatment increased the relative abundance of potentially beneficial bacteria while suppressing the abnormal proliferation of opportunistic pathogens. This study reveals the toxicological mechanism by which high concentrations of antibiotics (FF) disrupt the midgut microbiota balance in silkworms, induce tissue damage, and subsequently impede their growth and development. It also demonstrates for the first time that functional probiotics Bacillus velezensis LY5 effectively reverse this damage by restoring the silkworm midgut microbiota and intestinal tissue.