L-serine derived from the C-terminal region of Cry1Fa protoxin induces pathogenicity of an insect gut Enterococcus

The interactions between host, pathogen and gut microbiota can lead to outcomes in which the native bacteria can either enhance or attenuate the activity of the pathogen. Both have been reported for the microbiota of insects exposed to the Bacillus thuringiensis (Bt) toxins. In this study, we identified a specific Enterococcus strain that becomes dominant in the gut of Spodoptera frugiperda following damage induced by Cry1Fa protoxin, and which accelerates host mortality. This dominance is accompanied by significant alterations in gut metabolite profiles, particularly amino acid levels, and we found that L-serine enhances the adaptability of the Enterococcus in the injured gut, increasing its abundance by up to 100-fold, and its translocation into the hemocoel. Moreover, supplementation with L-serine significantly increased Cry1Fa protoxin-induced mortality. Notably, the C-terminal region of the protoxin, previously reported to exert minimal direct insecticidal activity, serves as a source of L-serine upon proteolytic cleavage in the gut. Both L-serine and hydrolysate from the digested protoxin were found to encourage biofilm formation in the Enterococcus strain, providing a potential mechanism for its enhancement of protoxin activity. Our findings reveal a previously unrecognized function of the C-terminal of Cry protoxin, which suggests potential for the toxicity enhancement.