Equol inhibits growth and spore formation of Clostridioides difficile

Equol is a nonsteroidal oestrogen of the isoflavone class. We investigated the antibacterial ability of equol with respect to the growth rate, toxin production and spore-forming abilities of Clostridioides difficile BI/027/NAP1.Isoflavones, or female hormones, were added to bacterial culture, which was grown at 35°C. The absorbance of the culture was measured at various time points for evaluating the growth inhibition. The toxin levels in the media and morphological changes were also assessed. To evaluate the influence of equol on the sporulation of C. difficile, cells were collected at various time points from the equol-supplemented culture and the number of spores was counted. Our results show that equol inhibits bacterial growth in a concentration-dependent manner. However, it does not inhibit the production of toxin by C. difficile. Other isoflavones and female hormones did not inhibit the C. difficile growth. At the 14th day, approximately 600 spores were present in the control medium and only six were seen in the equol-containing medium.Our results suggest that equol may directly inhibit the C. difficile growth in a concentration-dependent manner and spore formation.This is the first report on the antimicrobial ability of equol.