Assessing the role of folate syntrophy and folate cross-feeding in the pathobiology of infectious-inflamed milieu caused by Fusobacterium nucleatum

Diet and nutrition affect almost every biological process, including multiple chronic diseases, diabetes, and some cancers. However, there are still significant gaps in our understanding of the importance of nutrition and healthy diets in syntrophy with respect to cross-feeding of the microbe-microbe and the microbe-host in the pathobiology of the infectious-inflamed intestinal milieu caused by anaerobic opportunistic bacteria such as Fusobacterium nucleatum (F. nucleatum). We examined the immune outcomes of three-member folate syntrophy and cross-feeding between F. nucleatum bacteria, endogenous folate-producing gut bacteria, and host cells at the host-pathogen interface using a triple co-culture model. T84, THP-1, and Huh7 cells were inoculated with F. nucleatum for 6 h in regular DMEM, DMEM with 9.5 μM folic acid, or with/without a mixture of Bifidobacterium longum subsp. infantis (B. infantis) and Escherichia coli Nissle 1917 (EcN). Cytokine secretion, cometabolite levels (ammonia, indoles), cell viability, and barrier integrity were assessed. F. nucleatum-induced folate depletion was associated with increased IL-1β and IL-6 and decreased IL-22, along with reduced transepithelial electrical resistance (TEER) and cell viability in T84 cells. Folate supplementation mitigated these effects. The mixture of B. infantis and EcN reduced F. nucleatum-induced pro-inflammatory cytokines, increased IL-22, and improved TEER and cell viability. These protective effects were enhanced by the addition of folate. F. nucleatum also elevated ammonia and reduced indoles, effects reversed by B. infantis and EcN. In addition to the intrinsic pathogenicity of harmful bacteria, folate deprivation, microbe–microbe folate syntrophy, and microbe–host folate cross-feeding contribute to the pathobiology of anaerobic opportunistic bacteria and influence the physiological fate of host cells. A combination of B. infantis and EcN modulates the infectious-inflamed interface through a cytoprotective effect and mechanical competitive extrusion of pathogenic F. nucleatum. These results provide potential insights into the mechanisms of early-onset colorectal cancer, and evidently, require future studies using patient-derived organoids and in vivo systems to improve clinical relevance.