Oxygen-Dependent Nematicidal Activity of Bacillus velezensis YG4-2 Against Bursaphelenchus xylophilus and Identification of Active Metabolites

Pine wilt disease poses a serious threat to pine forest health, and efficient biocontrol agents are urgently needed. Bacillus velezensis YG4-2 is a pine endophytic bacterium with strong nematicidal activity against Bursaphelenchus xylophilus, but the effect of oxygen conditions on its activity remains unclear. In this study, we compared the growth, nematicidal activity, and extracellular metabolite profiles of YG4-2 under aerobic and anaerobic conditions. Aerobic cultivation significantly increased bacterial biomass and enhanced the nematicidal activity of both bacterial cultures and cell-free supernatants. LC-MS/MS-based untargeted metabolomics revealed clear metabolic differences between the two oxygen conditions, with 19 metabolites significantly upregulated under aerobic conditions. Activity validation showed that kynurenic acid exhibited the strongest nematicidal activity among the tested candidate metabolites. Targeted LC-MS/MS further confirmed the accumulation of kynurenic acid in aerobic culture supernatants, with an estimated concentration of 0.38 ng/mL, and no obvious signal was detected under anaerobic conditions. These results suggest that oxygen enhances the nematicidal activity of YG4-2 by promoting growth and regulating extracellular metabolite production, with kynurenic acid as a potential oxygen-responsive active metabolite.