Rates of perinatal HIV transmission have fallen substantially due to increased access to antiretroviral therapy during pregnancy and breastfeeding. However, this has led to a growing population of infants who are HIV- exposed but uninfected (iHEU). iHEU display heightened inflammation, immune activation and immune exhaustion potentially driven by their altered gut microbiota. HIV exposure has also been linked to impaired growth (stunting) in infants. Multiple studies have shown evidence of stunting in iHEU compared to HIV unexposed infants (iHU). In addition, malnutrition studies have linked chronic intestinal inflammation, which also associated with gut microbiota alteration, in infants to impaired growth. The mechanism(s) behind impaired growth in iHEU are not understood. It is plausible that heightened intestinal inflammation in iHEU may associate with stunting. However, whether the gut microbiota in iHEU causes impaired growth has not been formally tested. Beyond growth, the gut microbiota impacts immune development. Specifically, microbiota composition early in life imprint lasting immunological consequences. iHEU display high infectious morbidity including to enteric pathogens. For example, studies have shown that iHEU exhibit high prevalence of enteropathogenic Escherichia coli (EPEC) and Cryptosporidium spp compared to unexposed counterparts. The gut microbiota has been shown to protect against enteric pathogens via various mechanisms including colonization resistance and alteration of mucosal immunity. Whether gut microbiota in iHEU enhance susceptibility to enteric pathogens is unknown. Human studies are based on correlations which limit ability to infer causation. We will utilize a germ-free neonatal mouse model to investigate the causal role of the early stool microbiota in iHEU in driving these clinical phenotypes. In addition, we will use a neonatal mouse model of EPEC to test causality between stool microbiota and immunity to enteric pathogens. We hypothesize that the gut microbiota of iHEU early in life causes inflammation, poor growth and impairs immunity to enteric pathogens. We propose to test this hypothesis with the following specific aims. Aim 1: To investigate whether the early life gut microbiota of iHEU causes intestinal and systemic inflammation and impaired linear growth during infancy. Aim 2: To investigate whether the early life gut microbiota of iHEU impairs immunity to enteric pathogens