Isolation of Bacillus spp. from Thai fermented soybean (Thua-nao): screening for aflatoxin B1 and ochratoxin A detoxification.

AIMS

To study the interaction between Bacillus spp. and contaminating Aspergillus flavus isolated strains from Thai fermented soybean in order to limit aflatoxin production. To study the detoxification of aflatoxin B(1) (AFB(1)) and ochratoxin A (OTA) by Bacillus spp. in order to find an efficient strain to remove these toxins.

METHODS AND RESULTS

One A. flavus aflatoxin-producing strain and 23 isolates of Bacillus spp. were isolated from soybean and fresh Thua-nao collected from the north of Thailand. Inhibition studies of A. flavus and A. westerdijkiae NRRL 3174 (reference strain) growth by all isolates of Bacillus spp. were conducted by dual culture technique on agar plates. These isolates were also tested for AFB(1) and OTA detoxification ability on both solid and liquid media. Most of the strains were able to detoxify aflatoxin but only some of them could detoxify OTA.

CONCLUSIONS

One Bacillus strain was able to inhibit growth of both Aspergillus strains and to remove both mycotoxins (decrease of 74% of AFB(1) and 92.5% of OTA). It was identified by ITS sequencing as Bacillus licheniformis. The OTA decrease was due to degradation in OTalpha. Another Bacillus strain inhibiting both Aspergillus growth and detoxifying 85% of AFB(1) was identified as B. subtilis. AFB(1) decrease has not been correlated to appearance of a degradation product.

SIGNIFICANCE AND IMPACT OF THE STUDY

The possibility to reduce AFB(1) level by a strain from the natural flora is of great interest for the control of the quality of fermented soybean. Moreover, the same strain could be a source of efficient enzyme for OTA degradation in other food or feeds.