PROJECT SUMMARY Human milk contains essential nutrients and bioactives that are transferred to the nursing infant in what was once considered in a linear manner. Though now there is considerable evidence that human milk directs early establishment of the microbiome through molecules such as oligosaccharides that modulate specific microbial populations. Microbial communities that colonize the gastrointestinal tract enter into a commensal relationship with their host potentially impacting physiology. Accordingly, nitrogen bound in urea is delivered at relatively high concentrations in breast milk and may be liberated for utilization by the host and commensals by microbial urease activity. We hypothesize that urea nitrogen salvaging (UNS) is a key syntrophic feature of host-microbial interactions early in life. This may be of particular importance to infants in this critical stage of development, or in host populations where dietary nitrogen is limiting. This hypothesis will be addressed experimentally by evaluating and characterizing the metabolic capacity for infant-associated commensals to utilize urea and transform it to a usable form by their host. Moreover, we propose to study infant microbiome- mediated UNS modeled in an in vitro model to identify community-level phenomena. By understanding the impact to community structure and function by urea metabolism, we will define hallmarks of a microbiome that performs UNS efficiently. This study investigates a poorly understood and hypothetical host-microbial interaction with implications to nitrogen homeostasis early in development. The inter-kingdom UNS pathway may be of critical importance to infants in general or in certain nutritional contexts. In addition, this study further defines what constitutes a protective infant microbiome based on aggregate community function. This would potentially inform diagnostics to assess UNS capacity as well as develop interventions to correct suboptimal UNS. As such, purposeful modulation of UNS would increase the repertoire of tools to direct microbiome function while personalizing for life stage, diet, and/or host phenotype.