Aerotolerant Mechanism And Biotransformation Regulation Of Aerotolerant Mutagenesis Derived From Isoflavone Reduction Bacterium

Soy isoflavones (SI), which are mainly composed by genistein, daidzein, and glycitein, are secondary metabolites, being formed in the growing process of soy plants. Because they can combine with estrogen receptors and act just like estrogens, soy isoflavones are known as phytoestrogens. Soy isoflavones have a wide range of physiological functions, such as prevention and treatment of cancer, lipid lowering, antioxidant, prevention of osteoporosis arid the improvement of women's menopausal syndrome etc.. After being absorbed, soy isoflavone daidzein is able to be converted into different metabolites by gastrointestinal microflora, such as dihydrodaidzein (DHD), equol and O-desmethylangolensin (O-Dma), which are of higher and wider bioactivities. Strict anaerobic bovine rumen bacterial strain Niu-016, capable of biotransforming isoflavones daidzein into DHD, was used as intial strain for study. Strict anaerobic bacteria are very sensitive to oxygen existing in the environment, which make this kind of bacteria be unfittable for both scientific research and production in large scale. The oxygen tolerant mutant designated Aeroto-Niu-016 was obtained through oxygen tolerant domestication by increasing the redox potential of cultural medium gradually. The obtained oxygen tolerant strain is able to both grow and biotransform isoflavones daidzein and genistein in the presence of atmospheric oxygen. This is the first reported oxygen tolerant bacterium capable of biotransforming isoflavones.Our study revealed that strain Aeroto-Niu-016 became different from the native strain Niu-016 in many aspects, including a change in cell morphology (from rod to tangle), in propagation strategy (from binary fission to asymmetrically multibinary fission), in biophysico-biochemical traits (from indole negative to indole positive and from amylohydrolysis positive to negative), and point mutations in 16S rRNA gene (G398A and G438A). One reason why this Aeroto-Niu-016 is able to both grow and biotransform isoflavones daidzein and genistein into DHD and dihydrogenistein (DHG) in the presence of atmospheric oxygen may be due to the newly formed "protective coat". The "protective coat", which is mainly composed by exopolysaccharide, might prevent the normal transportation of oxygen through the membrane of the cell, the process of which would be beneficial for the aerobic growth of the oxygen-tolerant strain Aeroto-Niu-016. The biotransformation kinetics of daidzein by strain Aeroto-Niu-016 showed that the converting time got significantly shortened (from 40 h to 24 h), while the biotransforming capacity was decreased about 25% in comparison with that of the native strain Niu-016. The average bioconversion rate of daidzein and genistein by strain Aeroto-Niu-016 was 60.3% and 74.1%, respectively. And the maximum bioconversion capacity for daidzein was 1.2 mM, and 2.0 mM for genistein. Furthermore, when we added ascorbic acid (0.15%, m/v) in the cultural medium, the bioconversion rate of daidzein was increased from 60.3% to 71.7%, and that of genistein from 74.1% to 89.2%.