Fig. 2

Cultivated Lactobacillus secreted tryptophan metabolite norharman, which inhibited macrophage M1 activation in vitro. A Schematic diagram of LC–MS/MS. B A total of five Lactobacillus metabolites of tryptophan were identified by applying LC–MS/MS. C The effects of 5 metabolites on M1 macrophages were detected and norharman showed the strongest inhibitory effect by using qPCR (n = 5). D Norharman was mainly from tryptophan metabolism regulated by Lactobacillus by targeted LC–MS/MS (n = 4). E, F Norharman inhibited the activation of M1 macrophages, as shown by flow cytometry analysis (n = 3). Data are presented as the mean ± SEM; **P < 0.01 vs. the Ctrl group or tryptophan group; ^##P < 0.01 vs. the LPS group