LAUSR

BACKGROUND The soy isoflavone microbial metabolites dihydrodaidzein (DHD), dihydrogenistein (DHG), equol and 5-hydroxy-equol, are generally more biologically active than their precursors, daidzein and genistein. Bacteria responsible for isoflavone metabolism have been isolated and identified. Fermented soymilk is a potential functional food; however, there are few lactic acid bacteria capable of metabolizing soy isoflavones. RESULTS A newly isolated Gram-positive facultative anaerobic bacterium, which was named Lactobacillus acidipiscis HAU-FR7, was isolated from the traditional Chinese fermented soy product, "stinky tofu". Bacterium strain HAU-FR7 can grow under aerobic conditions and it can also convert most of the daidzin and genistin in soymilk into DHD and DHG, respectively. The concentrations of DHD and DHG produced were 183 and 134 μmol·L-1 , respectively, after fermentation for 24 h. Strain HAU-FR7 does not produce the biogenic amines, cadaverine, putrescine, histamine and tyramine and an antibiotic susceptibility test showed that HAU-FR7 is sensitive to nine of the ten tested antibiotics, except for vancomycin. Moreover, the DPPH free radical-scavenging capacity of soymilk fermented with HAU-FR7 was significantly higher than that of unfermented soymilk. CONCLUSION A facultative anaerobic lactic acid bacterium, designated Lactobacillus acidipiscis HAU-FR7 is capable of reducing the soy isoflavone glucosides, daidzin and genistin in soymilk to DHD and DHG efficiently, even in the presence of atmospheric oxygen. The biotransformation activity of HAU-FR7 grown in soymilk is higher than that in MRS liquid culture medium. This article is protected by copyright. All rights reserved.