Comprehensive analysis of pathogenic mechanisms of Enterobacter cancerogenus ECL9 in silkworms

Bacterial diseases of silkworms (Bombyx mori L.) cause significant reductions in sericulture and result in huge economic loss. Enterobacter cancerogenus is usually known as an opportunistic host pathogen. The interactions between pathogenic bacteria and silkworms involve complex processes. This study aimed to systematically investigate the genetic traits of E. cancerogenus ECL9 and its effects on the intestinal microbiota, as well as the physiological and immune responses of silkworm intestines to infection. Bacterial genome sequencing indicated that ECL9 possessed multiple antibiotic-resistant genes (including tetracycline, fluoroquinolone, and macrolide) and virulence genes (such as T6SS, RcsAB, Capsule, and AcrAB). To further evaluate the pathogenicity of this strain, silkworm larvae were subsequently infected. At 48 h post-infection, significantly elevated malondialdehyde level and reduced glutathione content were observed in the silkworm intestines, accompanied by decreased alkaline phosphatase activity. These biochemical changes were further corroborated by histopathological analysis, confirming the extent of intestinal damage. 16S rRNA gene sequencing analysis revealed that the composition of silkworm intestinal microbiota exhibited dynamic alterations at different time points post-infection with ECL9. Infection with E. cancerogenus ECL9 significantly altered the structure of the intestinal microbiota, although no significant changes were observed in alpha diversity. The family Enterobacteriaceae and the genus Enterococcus were identified as the key bacterial taxa driving these alterations. Quantitative real-time PCR (qRT-PCR) analysis revealed that significant up-regulation of attacin, cecropin, and lysozyme expression at 48 h post-infection with ECL9. Furthermore, correlation analysis indicated that alterations in the relative abundance of Enterobacteriaceae and Enterococcus were potential key factors contributing to the induction of these antimicrobial peptides (AMPs). This study enhances our understanding of the toxicity response of silkworms to E. cancerogenus infection.