Equol

The minimum inhibitory concentration (MIC) of the majority of isoflavone glycosides in soy (daidzin and genistin), their respective aglycones (daidzein and genistein), and the isoflavone metabolite equol (all from LC Laboratories, Woburn, MA, USA) were determined using a broth microdilution test, following standard procedures for aerobic [19 ] and anaerobic bacteria [20 ] with minor modifications. Briefly, individual colonies from the above plates were suspended in 5 mL of a sterile 0.9% NaCl solution (VWR International) to a McFarland turbidity of 1. The inoculated saline solution was then diluted 1:1000 in the test medium corresponding to the different species (see Table 1) to obtain an approximate final concentration of 3 × 10⁵ cfu mL⁻¹. Aliquots (100 μL) of the diluted cell suspensions were poured into microplate wells with 50 µL of two-fold increasing concentrations of the test compounds, ranging from 0.12 to 2048 µg mL⁻¹ (the limit of their solubility). MICs were established by visual inspection as the lowest concentration at which no visible growth was observed. All MIC assays were performed in duplicate. Where discrepancies between analyses were observed, a third assay was performed and the mode reported.