Abstract A large body of literature now indicates that antenatal and neonatal antibiotic exposure is associated with adverse childhood outcomes due to disruption of the developing microbiome. In premature infants, the standard of care for many decades has included the administration of broad-spectrum antibiotics in the first hours of life as treatment for a presumptive diagnosis of early onset sepsis. However, nearly all preterm infants receiving these antibiotics do not actually have sepsis. This grant renewal application proposes an ancillary microbiome study linked to the NANO (NICU Antibiotics and Outcomes) Trial, a recently launched clinical trial that will challenge this longstanding practice of immediately prescribing antibiotics to newborn preterm infants. NANO will test the hypothesis that antibiotics at birth worsens outcomes in preterm infants that are clinically stable. This multicenter trial, which is led by members of our research team, will randomize 802 premature infants to receive intravenous ampicillin and gentamicin or a saline placebo control. The study will measure the impact of variables including mode of delivery, gestational age, sex, and receipt of maternal milk, and administration of maternal antepartum antibiotics. Infant fecal samples in the first month of life as well as maternal fecal and vaginal swabs will be collected in NANO for basic microbiome profiling in the antibiotics and placebo groups using 16S rRNA gene sequencing. Here, we propose to augment microbiome analyses of NANO study subjects using novel strain-level metagenomic strategies and by analyzing samples beyond the first month of life. With this strategy, we propose to Aim 1. Test the hypothesis that empiric antibiotics (EA) disrupts mother-infant strain sharing in preterm infants. Aim 2. Test the hypothesis that EA increases the abundance of gut bacterial antimicrobial resistance genes in preterm infants. Aim 3. Test the hypothesis that EA delays the transition from a gut ecosystem dominated by facultative anaerobes to one dominated by obligate anaerobes. Because NANO is a first-of-its-kind clinical trial evaluating antibiotic therapy during the first days of life, this ancillary study will provide a rare opportunity to ask and answer a unique set of questions about the biology of early gut bacterial colonization.