PROJECT SUMMARY The human gut microbiota provides essential functions in human health. However, the microbiota is constantly subjected to challenges such as intestinal inflammation, which drives the microbiota into a perturbed state that can cause or exacerbate diseases. Therefore, microbial resilience, which maintains the structural and functional stability of the gut microbiome in the face of perturbations, is crucial to host health. Despite a central role in host health, the mechanisms underlying microbiota resilience remain poorly defined. During intestinal inflammation, host processes known as nutritional immunity starve gut microbes from essential micronutrients such as iron, constituting stress that both commensal and pathogenic bacteria must cope with to survive. Enteric pathogens overcome nutritional immunity using a series of exquisite mechanisms, including encoding receptors for host iron-binding proteins and producing iron-chelating molecules termed siderophores. In contrast to these well-studied strategies, how gut commensals survive iron starvation in the inflamed gut remains largely unknown. We propose that maintaining iron homeostasis is an essential strategy for commensals to remain resilient during gut inflammation. In preliminary studies, we demonstrated that the model gut commensal Bacteroides thetaiotaomicron (B. theta) acquires iron by pirating siderophores from an enteric pathogen that induces intestinal iron limitation. Our new preliminary data suggest that B. theta captures siderophores using an extracellular lipoprotein. We further show that enteric pathogens such as Salmonella can exploit this capture mechanism to “re-pirate” siderophores from gut commensals to evade nutritional immunity. In addition to increasing iron uptake, our unpublished data demonstrate that B. theta employs small, non-coding RNAs to orchestrate iron conservation to maintain intracellular iron homeostasis and combat nutritional immunity in the inflamed intestine. As such, our central hypothesis is that B. theta couples siderophore acquisition with small RNA-mediated intracellular iron conservation to maintain resilience in the inflamed gut. We will test our hypothesis by pursuing the following specific aims: 1) Elucidate how siderophore acquisition mediates B. theta resilience and modifies host nutritional immunity in the inflamed gut; and 2) Determine how B. theta maintains intracellular iron homeostasis in the inflamed gut. The completion of this work will reveal the mechanisms by which gut commensals adapt to iron limitation and how such adaptation shapes the structural and functional stability of the microbiota during gut inflammation. This research is innovative because it adds commensal iron metabolism as a previously unrecognized dimension to the intricate interactions between pathogen and nutritional immunity. This proposed work is impactful because establishing a model for iron regulation in B. theta will provide insights into how interp...