Effect of Soy Isoflavones on Growth of Representative Bacterial Species from the Human Gut

The present work aimed to assess the susceptibility of dominant and representative bacterial populations from the human gut to isoflavones and their metabolites. To do so, the minimum inhibitory concentration (MIC) of isoflavone glycosides, isoflavone aglycones, and equol to 37 bacterial strains was determined by broth microdilution. Additionally, for 10 representative strains, growth curves, growth rate (μ), and optical density (OD(600 nm)) of the cultures at 24 h were also determined. MICs of daidzin, genistin, daidzein, and genistein were >2048 μg mL(−1) for all strains assayed, while that of equol ranged from 16 μg mL(−1) for Bifidobacterium animalis subsp. animalis to >2048 μg mL(−1) for Enterobacteriaceae strains. Changes in growth curves, μ, and final OD were observed among the species in the presence of all tested compounds. Genistein reduced μ of Bacteroides fragilis, Lactococcus lactis subsp. lactis, and Slackia equolifaciens, while both genistein and equol increased that of Lactobacillus rhamnosus and Faecalibacterium prausnitzii. Compared to controls, lower final OD in the presence of aglycones and equol were recorded for some strains but were higher for others. Altogether, the results suggest that isoflavone-derived compounds could modify numbers of key bacterial species in the gut, which might be associated with their beneficial properties.