BACKGROUND Monensin is widely used in livestock and poultry production to promote animal growth and control coccidiosis. Most monensin is excreted as unchanged parent compounds via feces. However?monensin is very stable and difficult to degrade in the environment, and is potentially risky for the health of wildlife and humans. To date, the biodegradation process of monensin by bacteria and its mechanism is unclear. In this study, we found that a Stenotrophomonas maltophilia DM-2 strain isolated from chicken manure could effectively degrade monensin. RESULT The optimum temperature and pH for DM-2 to degrade monensin were 40 degrees C and pH 7.0. A potential degradation pathway of monensin was proposed based on the identification and characterization of two new monensin metabolic intermediates by liquid chromatography quadrupole time-of-flight mass spectroscopy. In addition, an inducible monensin-degrading activity was present in DM-2. A soluble epoxide hydrolysis (sEH) gene highly expressed in the DM-2 on day 3 and 6 under monensin stress, which was not observed in the DM-2 without monensin stress. When a specific inhibitor TPPU (1-(1-propanoylpiperidin-4-yl)-3-[4-(trifluoromethoxy)phenyl]urea) of sEH was added into the cell-free extract, the degradation of monensin was inhibited. CONCLUSION Strain DM-2 can biodegrade monensin by soluble epoxy hydrolase and produce two new intermediates. To our knowledge, this is the first report of the degradation of monensin by Stenotrophomonas strains. The novel metabolic pathways of monensin which we found may provide new insight into the environmental fate of monensin. (c) 2020 Society of Chemical Industry