Gut inflammation constitutes a growing health concern in developed countries. It coincides with dysbiosis, including a lack of anti-inflammatory bacteria. As an example, propionibacteria are lacking in the microbiota of newborns which develop necrotizing enterocolitis. We investigated immunomodulatory properties of Propionibacterium freudenreichii.A screening led to the selection of P. freudenreichii CIRM-BIA129, a strain inducing high levels of regulatory IL-10 in human PBMCs. Consumption of this strain protected mice from colitis induced either by TNBS or by DSS. It alleviated severity of symptoms, modulated local and systemic inflammation, as well as colonic oxidative stress and epithelial cell damages. It further mitigated severity of mucositis induced by 5-fluorouracyl, preventing weight loss, reducing inflammation and mucosal damages.Mutation of the slpB gene, encoding a key surface layer protein, suppressed this immunomodulatory effect and the resulting slpB mutant induced a rather proinflammatory response and failed to prevent mucositis. Accordingly, consumption of Lactococcus lactis NCDO 2118 harboring pXIES-SEC:slpB and expressing the propionibacterial SlpB reduced severity of colitis, lowered weight loss, disease activity index, shortening of the colon length, and histopathological score, compared with mice treated with L. lactis NCDO 2118 wild-type strain.P. freudenreichii was further shown to produce extracellular vesicles (EVs), which mimic the immunomodulatory features of propionibacteria in vitro by modulating NF-κB transcription factor activity and IL-8 release in cultured human intestinal epithelial cells (HIEC). Proteomic analysis revealed presence of surface layer (S-layer) proteins, including SlpB, in these EVs. Guanidine treatment of P. freudenreichii intact cells leads to extraction of surface proteins, which constitute the S-layer, the outmost structure of bacteria. These extracted proteins also mimic the effect of propionibacteria, inducing IL-10 in immune cells and modulating NF-κB and IL-8 in HIEC.This work emphasizes the importance of extractable surface proteins, including SlpB, and of EVs, in P. freudenreichii probiotic effects. It opens perspectives for the development of probiotic and postbiotic products aimed at decreasing inflammation.