Updated project metadata. Opioid use disorder (OUD) is a public health crisis currently being exacerbated by increases in use of fentanyl; therefore, the identification of novel biomarkers and treatment strategies is critical. Here, we define how manipulations of the gut microbiome drive fentanyl intake, fentanyl seeking, and alter proteomic plasticity in the nucleus accumbens. These findings establish clear relevance for gut-brain signaling in OUD, and lay foundations for further translational work in this space.