The newly identified bacterium Dysosmobacter welbionisJ115(T) improves host metabolism in high-fat diet (HFD)-fed mice. To investigate mechanisms, we used targeted lipidomics to identify and quantify bioactive lipids produced by the bacterium in the culture me-dium, the colon, the brown adipose tissue (BAT), and the blood of mice. In vitro, we compared the bioactive lipids produced by D. welbionisJ115(T) versus the probiotic strain Escherichia coli Nissle 1917. D. welbionis J115(T) admin-istration reduced body weight, fat mass gain, and improved glucose tolerance and insulin resistance in HFD-fed mice. In vitro, 19 bioactive lipids were highly produced by D. welbionisJ115(T) as compared to Escherichia coli Nissle 1917. In the plasma, 13 lipids were significantly changed by the bacteria. C18-3OH was highly present at the level of the bacteria, but decreased by HFD treatment in the plasma and normalized in D. welbionis J115(T)-treated mice. The metabolic effects were associated with a lower whitening of the BAT. In the BAT, HFD decreased the 15-deoxy-Delta 12,14-prostaglandin J2, a peroxisome proliferator-activated receptor (PPAR-gamma) agonist increased by 700% in treated mice as compared to HFD-fed mice. Several genes controlled by PPAR-gamma were upregulated in the BAT. In the colon, HFD-fed mice had a 60% decrease of resolvin D5, whereas D. welbionisJ115(T)-treated mice exhibited a 660% increase as compared to HFD-fed mice. In a preliminary experiment, we found that D. welbionis J115(T) improves colitis. In conclusion, D. welbionis J115(T) influences host metabolism together with several bioactive lipids known as PPAR-gamma agonists.