The possible mechanisms of antimutagenicity against 4-nitroquinoline-N-oxide (4-NQO; a direct mutagen) and 3,2′-dimethyl-4-amino-biphenyl (DMAB; an indirect mutagen) were examined in fermented soymilk prepared with a coculture of Streptococcus thermophilus and Bifidobacterium infantis. The antimutagenicity in the fermented soymilk was not due to the bioantimutagenic effect of modulation of DNA repair processes. The mutagenicity of DMAB decreased with increased pre-incubation of fermented soymilk and the DMAB metabolite but not with intact DMAB or an S9 mixture. Mutagenicity of 4-NQO was not affected by preincubation of fermented soymilk with this mutagen. Mutagenicity of both 4-NQO and DMAB was reduced when Salmonella Typhimurium TA 100 was pretreated with fermented soymilk, indicating that fermented soymilk affected the function of the bacterial cell, which might also lead to reduced mutagenicity of the tested mutagens. Desmutagenic and blocking effects were the main mechanisms of antimutagenicity in the fermented soymilk against DMBA. In contrast, the antimutagenic effect of the fermented soymilk on 4-NQO was primarily due to a blocking effect.

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