Colonizing Bacteria Aggravate Inflammation, Cytotoxicity and Immune Defense During Influenza A Virus Infection.

A diverse bacterial community colonizes the respiratory system, including commensals such as and , as well as facultative pathogens like . This study aimed to establish a colonized cell culture model to investigate the impact of these bacteria on influenza A virus (IAV) infection. Respiratory epithelial cells were exposed to , , or , using either live or heat-inactivated bacteria, followed by IAV infection. Cell integrity was assessed microscopically, cytotoxicity was measured via LDH assay, and inflammatory responses were analyzed through cytokine expression. Additionally, macrophage function was examined in response to bacterial colonization and IAV infection. While commensals maintained epithelial integrity for 48 h, induced severe cell damage and death. The most pronounced epithelial destruction was caused by coinfection with and IAV. Notably, commensals did not confer protection against IAV but instead enhanced epithelial inflammation. These effects were dependent on live bacteria, as inactivated bacteria had no impact. However, prior exposure to and improved macrophage-mediated immune responses against IAV. These findings suggest that while individual commensals do not directly protect epithelial cells, they may contribute to immune training and enhance lung defense mechanisms.