A Dominant Probiotic from the Traditional Chinese Fermented Food Douzhier Exhibits Anti-Influenza Activity

Background: Douzhier, a traditional Chinese fermented food with a history spanning over 1000 years, harbors a complex community of beneficial microbiota. In this study, we isolated Lactococcus lactis subsp. cremoris (designated L. cremoris D2022), one of the dominant bacterial species in Douzhier, and evaluated its efficacy against influenza A virus (IAV) infection. Methods: A murine model of IAV infection was established using C57BL/6J mice. Following oral administration of L. cremoris D2022, treated and model control groups (IAV-infected model control group received phosphate-buffered saline (PBS)) were comparatively assessed through clinical outcomes (body weight dynamics and survival), pathological parameters (colon length, lung index, and lung histopathology), inflammatory cytokine profiling, multi-omics analyses (lung transcriptomics and cecal metabolomics), and adaptive immune responses. Results: Administration of L. cremoris D2022 markedly alleviated IAV-induced pathology compared with PBS-treated model controls, as evidenced by improved survival (30% vs. 0%, p = 0.006), reduced body weight loss, decreased lung index, and preservation of colon length. D2022 treatment accelerated viral clearance, accompanied by enhanced early interferon beta (IFN-β) production. This was followed by suppression of late-stage hyperinflammation, including reduced levels of interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor alpha (TNF-α) at 7 dpi. Metabolomic analysis revealed increased levels of deaminotyrosine (DAT), short-chain fatty acids (SCFAs), and microbially conjugated bile acids (e.g., ursodeoxycholic acid (UDCA) and isoallolithocholic acid (isoalloLCA)). Furthermore, IAV-specific adaptive immune responses were significantly enhanced. Conclusions: L. cremoris D2022 protects mice against IAV infection, with enhanced early IFN-β responses, reduced late-stage inflammation, and associated changes in intestinal microbial metabolites. The causal roles of these metabolites remain to be established. These findings support further evaluation of D2022 as a probiotic candidate for adjunctive influenza prevention.