Identification of AFB1-degrading strain Bacillus subtilis HQ3 and its biodegradation mechanism

Aflatoxin B1 (AFB1) is one of the most hazardous mycotoxins, and occurrences in food, feed, and their raw materials, presenting risks to the health of humans, animals and environment. Thus, study on the biodegradation of AFB1 is imperative to tackle this problem. In this study, by means of sequence analysis and phylogenetic tree construction of 16S rDNA, five strains (HQs) with the ability to degrade AFB1 were identified. These strains showed more than 99 % identity with Bacillus subtilis. When cultivated in LB medium, the HQs strains were able to degrade 72.13 %–81.34 % of AFB1 at 50 μg L⁻¹. Notably, strain HQ3 could effectively degrade 86.58 % of AFB1 at 50 μg L⁻¹ in LB medium after 4 days incubation at 35 °C, pH 9.0, and a C/N ratio of 3/2. Under optimal conditions, it could degrade 88.32 % of AFB1 at 50 μg kg⁻¹ in peanut meal.

Further analyses of the genetic function showed that strain HQ3 harbored genes encoding AFB1-degrading enzymes, such as multi-copper polyphenol oxidoreductase laccase (HQ3-Laccase) and myo-inosose-2 dehydratase (HQ3-MID). The enzymes in the cell-free supernatant of HQ3 played a major role in AFB1 degradation, with a degradation of 80.52 %. Moreover, the sequence of HQ3-Laccase exhibited 99.64 % similarity to that of CotA laccase from B. amyloliquefaciens, and HQ3-MID showed 99.49 % similarity to that of BADE from B. subtilis. The expression of the two enzyme-encoding genes increased as the AFB1 content increased from 0 to 20 or 40 μg L⁻¹. These findings demonstrated that AFB1 could be removed by HQ3-laccase and HQ3-MID.