Negative controls for microbiota analysis Raw sequence reads. Negative controls for microbiota analysis

Commensal microbiota play an important role in many physiological processes. Importantly, they pose a natural threshold to colonization with pathogenic bacteria by direct competition. Here we show that acute infection of the lung with influenza A viruses causes elimination of a substantial proportion of bacteria from the small intestine. This weakens the natural shield against pathogenic bacteria and renders the intestinal epithelium vulnerable for invading bacteria. We further show that influenza A virus infection up-regulates the activity of Paneth cells a major source of antimicrobial peptides in the small intestine. This suggests that influenza A viruses affect a host mechanism of microbiota regulation to cause bacterial depletion in the small intestine. Surprisingly, the same mechanism does not apply in the respiratory tract, where influenza A virus infection causes subtle qualitative but not quantitative changes in the commensal microbiota. Overall, our date provide the first description of microbiota dynamics in an acute infection animal model.