CLDN3 and mastitis: We also showed that disrupting the intestinal microbiota impaired AhR activation, and deteriorated E. coli-induced barrier injury by reducing occludin and claudin-3 expression (Fig 4G–4J). Collectively, these results indicate that altering the intestinal microbiota could impair AhR activation and be associated with mastitis initiation and the development of E. coli-induced mastitis.