The natural symbiotic bacterium Enterococcus faecalis LX10 drives Bombyx mori refractoriness to Nosema bombycis infection via the secretion of enterococcin.

Background: The microsporidian Nosema bombycis is an obligate intracellular fungal-related parasites of the Bombyx mori, causing the epidemic disease Pebrine and extensive economic losses in the agricultural and sericulture industry. Enterococcus has emerged as one of the predominant gut microorganisms of the major model organism, Bombyx mori. However, the potential interactions mechanism between B. mori, N. bombycis and Enterococcus have not been well demonstrated.

Methods: To address this gap, we used an insect model, silkworm to examine the potential mechanism of the natural symbiotic bacterium Enterococcus faecalis LX10 drives B. mori refractoriness to N. bombycis infection. E. faecalis LX10 was isolated from the gut of healthy silkworms, and its inhibitory activity against N. bombycis was evaluated at both the cellular and individual levels using posttranslational modifications, gene and protein expression analysis, transfected cells, and in vitro immunofluorescence.

Results: We demonstrated that enterococcin (EntLX), the first antimicrobial protein family in gut commensal bacterium Enterococcus faecalis LX10 of B. mori, contributes to defending against N. bombycis infection resistance depends on the enzyme gelatinase (GelE), disulfide bond and disulfide bond formation proteinA (DsbA). The EntLX protein, abundantly expressed in transgenic BmN cells and gut organs(gut epithelium, peritrophic membrane and contents), can reduce the infection rate of cells and alleviate intestinal damage caused by N. bombycis infection. After simultaneous vaccination with E. faecalis LX10 and N. bombycis, the differentially key metabolites, physiological characteristics(larval mass), or economic traits(cocoon length, cocoon width, whole-cocoon weight, cocoon shell weight, pupation rate and adult emergence rate) showed a certain degrees of recovery and correction compared with those of single N. bombycis inoculation at the individual level.

Conclusions: This study advances the understanding of the anti-microsporidia activity of enterococci and paves the way for the expression of these molecules as antifungal agents via the genetic transformation of Enterococcus symbionts from disease-transmitting insects.

Clinical trial number: Not applicable.