Approximately 40,000 infants are born each year in the United States with congenital heart defects (CHD), and heart defects are the leading cause of birth defect-related deaths in the United States. While advances in surgical treatment and cardiopulmonary bypass have improved survival for children born with CHD, these infants continue to suffer morbidity from post-operative multiple organ dysfunction (MODS)2,26. Infants with left sided obstructive CHD are at particular risk of MODS. Putative mechanisms for post-operative MODS are via loss of intestinal epithelial barrier function (EBF) and low intestinal microbiome diversity9,15,27. In a single center pilot study of 20 infants and children after surgical repair of CHD, we found that delayed enteral nutrition (EN) initiation was associated with worse intestinal epithelial integrity. We also identified that longer duration of post- operatve antibiotics was associated with loss of tight junction integrity and worse intestinal permeability, perhaps through reduced intestinal microbiome diversity 22,23,28. These lines of evidence suggest that delayed EN initiation and longer duration of post-operative antibiotic prophylaxis may increase risk for MODS in infants after surgical correction of left sided obstructive CHD through negative effects on the host intestinal EBF and microbiome diversity. The goal of this proposal is to prepare Dr. Typpo for an independent career in clinical- translational research investigating the role intestinal EBF and microbiome diversity on MODS incidence and severity through an educational program and completion of the following 2 specific aims: Specific Aim 1: Determine the effect of timing and composition of EN on the recovery of intestinal EBF over the first 7 post- operative days for infants undergoing surgical correction of left sided obstructive cardiac defects. Specific Aim 2: Determine the effect of duration and type of perioperative antibiotic exposure on change from baseline microbiome diversity, intestinal EBF, and site-specific intestinal permeability over the first 7 post-operative days in infants undergoing surgica correction of left sided obstructive cardiac defects. We will leverage existing practice variation for timing of EN initiation and antimicrobial prophylaxis across three cardiac centers. This proposal is significant, because we will address gaps in knowledge regarding the optimal timing of EN initiation after surgical correction of infant CHD to maintain intestinal barrier functions. This proposal is innovative, because we will apply knowledge gained from animal models to investigate how duration of perioperative antibiotics and EN timing and composition might negatively alter the intestinal microbiome for human children. Care strategies with the intent to limit negative effects on the intestinal microbiome during a critical window of microbiome development may have long lasting effects on the development of several chronic illnesses, in addition to immediate effects on risk of MODS25,30-32,80,81. This career development award will prepare Dr. Typpo to apply the emerging science of minimally invasive intestinal EBF assessment and microbiome analysis to identify strategies to reduce post-operative MODS in her patients.