Degradation of aflatoxin B1 from naturally contaminated maize using the edible fungus Pleurotus ostreatus

Abstract

Aflatoxins are highly carcinogenic secondary metabolites that can contaminate approximately 25% of crops and that cause or exacerbate multiple adverse health conditions, especially in Sub-Saharan Africa and South and Southeast Asia. Regulation and decontamination of aflatoxins in high exposure areas is lacking. Biological detoxification methods are promising because they are assumed to be cheaper and more environmentally friendly compared to chemical alternatives. White-rot fungi produce non-specific enzymes that are known to degrade aflatoxin in in situ and ex situ experiments. The aims of this study were to (1) decontaminate aflatoxin B₁ (AFB₁) in naturally contaminated maize with the edible, white-rot fungus Pleurotus ostreatus (oyster mushroom) using a solid-state fermentation system that followed standard cultivation techniques, and to (2) and to assess the risk of mutagenicity in the resulting breakdown products and mushrooms. Vegetative growth and yield characteristics of P. ostreatus were not inhibited by the presence of AFB₁. AFB₁ was degraded by up to 94% by the Blue strain. No aflatoxin could be detected in P. ostreatus mushrooms produced from AFB₁-contaminated maize. Moreover, the mutagenicity of breakdown products from the maize substrate, and reversion of breakdown products to the parent compound, were minimal. These results suggest that P. ostreatus significantly degrades AFB₁ in naturally contaminated maize under standard cultivation techniques to levels that are acceptable for some livestock fodder, and that using P. ostreatus to bioconvert crops into mushrooms can reduce AFB₁-related losses.

Keywords: Aflatoxin; Biodegradation; Pleurotus ostreatus; Solid-state fermentation.

Fig. 1

Degradation of AFB₁ by P. ostreatus. Box plots show the percentage of AFB₁ degraded (left axis) by P. ostreatus strains grown on uncontaminated (0 ng/g) or contaminated maize at 25, 250 and 2500 ng g⁻¹ AFB₁, respectively. Bars show raw AFB₁ concentrations (right axis) of treated (N001, Pearl and Blue) or untreated (mock) samples. Whiskers on box plots show the minimum and maximum values and means are indicated by dashed lines. Box plot values not connected by the same letter are significantly different (p ≤ 0.05) by Tukey’s HSD. Whiskers on bars indicate standard errors. Asterisks above bars show significant difference (p ≤ 0.05) by Tukey’s HSD of treated values compared to controls of the same initial AFB₁ concentration

Fig. 2

Mutagenicity of breakdown products from AFB₁-contaminated maize treated with P. ostreatus. Number of revertant CFUs when exposed to the extraction solvent without AFB₁ (MeOH), or extracts from untreated (mock) and treated (N001, Pearl and Blue) maize with initial AFB₁ concentrations of 2500 ng g⁻¹, and from uncontaminated (0 ng g⁻¹) maize. Whiskers show standard errors