Immune-mediated indirect interaction between gut microbiota and bacterial pathogens

Background Organism survival depends on the ability to orchestrate interactions between the host immune system and gut microbiota in response to pathogenic infections. These tripartite interactions shape pathogen virulence evolution. A key regulator of the immune system and, hence, bipartite interactions (in insects) is the immune deficiency (Imd) pathway, which modulates gut microbiota and pathogens by synthesizing antimicrobial peptides (AMPs). However, whether Imd-dependent AMPs mediate indirect interactions between gut microbiota and pathogens in a tripartite context remains unclear. Using RNAi-mediated knockdown of Tenebrio molitor Relish ( TmRelish ), we hypothesized that Imd-dependent AMPs influence indirect interaction between Providencia burhodogranaria_B infection and the gut microbiota. Results While TmRelish -knockdown altered bipartite interactions, disrupting gut microbiota load and composition and increasing pathogen load and virulence through higher host mortality, it did not support our tripartite hypothesis. Instead of Imd-dependent AMPs, gut microbiota and pathogen were influenced by Imd-independent AMPs expression, suggesting alternative regulatory pathways. Nevertheless, our investigations of tripartite interactions showed a positive effect of P. b_B infection on gut microbiota load, while survival analysis further revealed a negative effect of gut microbiota on pathogens infection, suggesting microbiota- mediated immune priming. Conclusions These findings suggest that while Imd-dependent AMPs may not mediate tripartite interactions, microbiota-host interactions, such as microbiota-mediated immune priming and changes in microbiota load, can shape infection outcomes. These effects on infection outcomes almost certainly exert important selective pressures on the evolution of bacterial virulence.