English abstract
Aflatoxin contamination in food products is a serious global concern due to its well documented carcinogenic, mutagenic and immunosuppressive effects on human health. The aflatoxins are primarily produced by Aspergillus flavus and A. parasiticus as secondary metabolites when they grow on agricultural commodities such as maize, groundnut, chilli, tree nuts and rice. Among the various types of aflatoxins, aflatoxin B1 (AFB1) poses the greatest threat to humans. People are exposed to aflatoxin directly by ingesting contaminated food or indirectly through animal-source foods such as milk, eggs, and meat derived from animals previously exposed to aflatoxins in their feed. Eradicating or removing aflatoxin after its formation in an agricultural commodity is challenging due to its high stability. To minimize the harmful effects of aflatoxin, a variety of detoxification procedures are employed. The use of microorganisms is considered a practical biological approach to reduce aflatoxin levels in foods and feeds. The microorganisms achieve this reduction through either physical binding or enzymatic degradation of aflatoxin. In general, the insects feeding on AFB1-contaminated cereal grains are unaffected. It was hypothesized that bacteria residing in the gut of such insects may possess the capacity to degrade/bind aflatoxin and reduce its toxic effects. In this study, bacteria residing in the gut of the rice weevils (Sitophilus oryzae L.) (Coleoptera: Curculionidae) feeding on aflatoxin-contaminated corn kernels were isolated and evaluated for their ability to suppress Aspergillus flavus and to remove/degrade AFB1. Four morphologically distinct S. oryzae gut-associated bacterial isolates were isolated and identified as Bacillus subtilis (RWGB1), Bacillus oceanisediminis (RWGB2), Bacillus firmus (RWGB3) and Pseudomonas aeruginosa (RWGB4) based on 16S rRNA gene sequence analysis. These bacterial isolates inhibited A. flavus growth in the dual culture assay and induced morphological deformities in the fungal hyphae, as confirmed by scanning electron microscopy. All four bacterial isolates were capable of removing AFB1 from the nutrient broth medium. In addition, culture supernatants of these bacterial isolates degraded AFB1, and the degradation of toxin molecule was confirmed by liquid chromatography-mass spectrometry. The bacterial isolates, B. subtilis RWGB1, B. oceanisediminis RWGB2 and P. aeruginosa RWGB4, were capable of producing antifungal volatile organic compounds that inhibited A. flavus growth. These results suggest that the bacterial isolates from S. oryzae gut have the potential to bind and/or degrade AFB1. Further research on their application in the food and feed industries could enhance the safety of food and feed production.
