AbstractIn adult animals, acute viral infections only temporarily alter the composition of both respiratory and intestinal commensal microbiota, potentially due to the intrinsic stability of this microbial ecosystem. In stark contrast, commensal bacterial communities are rather vulnerable to perturbation in infancy. Animal models proved that disruption of a balanced microbiota development e.g., by antibiotics treatment early in life, increases the probability for metabolic disorders in adults. Importantly, infancy is also a phase in life with high incidence of acute infections. We postulated that acute viral infections in early life might pose a similarly severe perturbation and permanently shape microbiota composition with long-term physiological consequences for the adult host. As a proof of concept, we infected infant mice with a sub-lethal dose of influenza A virus. We determined microbiota composition up to early adulthood (63 days) from small intestine by 16S rRNA gene-specific next-generation sequencing. Infected mice underwent long-lasting changes in microbiota composition, associated with increase in fat mass. High-fat-high-glucose diet promoted this effect while co-housing with mock-treated animals overwrote the weight gain. Our data suggest that in the critical phase of infancy even a single silent viral infection could cast a long shadow and cause long-term microbiota perturbations, affecting adult host physiology.