This revised BLRD and CSRD Collaborative Merit Award (BCCMA) application is being submitted as part of a group of 5-linked CMAs from nationwide VA investigators in gastrointestinal (GI) and liver diseases, who formed a national steering committee after participating in a successful field-based meeting in San Diego in May 2019 (funded by VAORD). The roadmap developed at this meeting was published in the journal “Gastroenterology” (PMC7249241). This cluster of CMAs is aimed at in depth understanding of the emerging role of gut microbiome in the pathophysiology of Veterans deployment related GI and liver diseases including the Gulf War Illness (GWI), Post Traumatic Stress Disorder (PTSD) and Inflammatory Bowel Diseases (IBD), and to develop potential biotherapeutics to alleviate the disease symptoms. This specific proposal CMA1, is driven by the facts that while diarrheal disorders are highly prevalent in Veterans with GWI and are a major cause of high morbidity, the pathophysiology of GWI is not well-defined, and the treatment options are inadequate. Therefore, in view of the emerging role of the gut microbiome in GWI, there is a critical need to better understand the role of microbial dysbiosis and the mechanisms involved in higher incidence of diarrheal disorders and IBS in these Veterans. Recent studies have further highlighted the crucial role of gut microbiome in susceptibility to diarrheal pathogens. For example, gut microbiome has been shown to be the key determinant in susceptibility to infection and diarrhea by C. rodentium (CR, murine counterpart of the human enteropathogenic E. coli). Additionally, fecal microbial transplant (FMT) based treatment strategies for C. difficile infection further support the critical role of gut microbiome in diarrheal diseases. However, direct causal relationship of microbial dysbiosis observed in GWI Veterans and diarrheal illnesses remains uninvestigated. In this regard, in preliminary studies we established a humanized GWI mouse model where FMT from GWI Veterans in mice for 14 days almost recapitulated some features of the functional bowel disorders associated with GWI, including a) significant dysregulation in intestinal barrier integrity; and b) mild inflammation as assessed by neutrophil infiltration. Another novel and striking finding in the humanized GWI mice was a marked decrease in colonic expression of the key chloride transporter (DRA, Down Regulated in Adenoma). Of note, decreased DRA expression is a key event in infectious or IBD associated diarrhea and is also linked to compromised barrier integrity. Also, in preliminary studies, we observed that DRA KO mice were much more susceptible to CR infection. Based on these data, we hypothesize that “Dysbiosis in GWI Veterans is associated with compromised barrier integrity and decreased DRA expression which contributes to increased predisposition of veterans to pathogen infections and diarrhea. We propose to utilize state-of- the-art animal models e....