Environmental Degradation of Aflatoxins and Genetic Diversity of Aspergillus Section Flavi with a Focus on Mozambique

Abstract

Aflatoxins are potent hepatotoxic, carcinogenic, genotoxic secondary metabolites produced by species in Aspergillus section Flavi that contaminate foods and feeds worldwide. Aflatoxin contamination of staple crops is endemic in Mozambique, and the country has one of the highest rates of liver cancer in the world. Groundnut exportation has been reduced due to contamination above the regulatory limits. However, there are few studies that detail the etiological agents of aflatoxin contamination in Mozambique. In addition, both in developed and developing countries there is a need for sustainable methods to manage crops already contaminated with aflatoxin in the environment of which, the practice of plowing crops under the soil to manage large fields with high levels of aflatoxin has raised concerns about the persistence in the soil of aflatoxins and the aflatoxin-producing fungi of the crop debris. In chapter one, a literature review of aflatoxins, important hosts and taxonomic classification of the major aflatoxin producing fungal groups is presented. Furthermore, the role of the environment, methods of control and management of aflatoxin is discussed. In chapter two, an analysis of fungi of S morphology isolated from maize and groundnuts from the northern region of Mozambique is presented. Fungi of the S morphology generally produce very high levels of aflatoxin and are important causal agents of contamination. A polyphasic approach was taken combining morphological and phylogenetic analysis, comparing the Aspergillus section Flavi fungi of Mozambique with previously described species with similar morphology. This defined the S morphology isolates of Mozambique as members of an A. minisclerotigenes complex. Within the complex, a group of these Mozambique isolates with a 30bp aflR deletion, resulting in an internal 10 amino acid deletion, are potentially important causal agents of aflatoxin contamination of maize and groundnuts. These isolates produce both B and G aflatoxins. These insights provide the basis for understanding the aflatoxin contamination of crops in Mozambique. The findings of this chapter will be submitted for publication in the journal Phytopathology. In chapter three, a new mechanism by which commercial atoxigenic A. flavus biocontrol agents limit aflatoxin levels in crops is presented. We report that these agents are able to degrade up to 82% of the aflatoxins in contaminated maize. Degradation by these isolates was independent of the presence or absence of aflatoxin biosynthesis cluster genes, in contrast to previous data indicating an association of aflatoxin degradation with an isolates’ ability to produce aflatoxin. This insight will provide methods for in crop assessment of degradation and identifies an additional trait for the selection of optimal atoxigenic biocontrol isolates. The results of this chapter has been submitted for publication in the journal Toxins. In chapter four, the persistence in soil of aflatoxins and aflatoxin-producing fungi, following the incorporation of a contaminated crop into the soil is determined. An in vitro analysis suggests that aflatoxins in contaminated crops are significantly degraded and the Aspergillus section Flavi population is reduced when incorporated into the soil via both soil chemical processes and the presence of soil microbes. The observations of this chapter will be submitted as a short communication to the journal Soil Microbiology and Biochemistry. The results of these chapters contribute to insights of etiological agents of contamination of crops in Mozambique and methods to control and manage aflatoxins of contaminated crops to help in the development of rational farming practices that result in improved food safety.