EFFECT OF GAMMA IRRADIATION ON ASPERGILLUS FLAVUS AND SOME OF ITS TOXIC METABOLITES

Abstract

Studies on the resistance of aflatoxin B1, B1 + B2, G1 and G1 + G2 against gamma irradiation in the dry state and when dissolved in chloroform or methanol were carried out by means of thin-layer chromatography, fluorescence spectrophotometry, and optical density retention. The aflatoxins particularly in the dry state were fairly stable against gamma irradiation: the mixture of aflatoxins G1 + G2 was the most resistant. Resistance was least when the aflatoxins were irradiated in methanol. Probable alterations in the toxic properties of aflatoxins B1, B1 + B2, G1 and G2 due to gamma irradiation weretested with yolk inoculation of 10-day old chick embryos. The results of sub-lethal doses revealed that aflatoxin toxicity decreased with incease in the gamma irradiation does level. Histopathological examination of liver of the hatched chicks injected with various concentrations of irradiated and non-irradiated aflatoxin B1, B1 + B2, G1 and G1 + G2 were also carried out and compared with control animals injected with carrier solvent. The livers showed fatty infilteration, hyperaemia and necrosis.. the severity of the lesions caused by the aflatoxins decreased with increased gamma irradiation dosage of the aflatoxins. Daily variations in toxin production by the exposure of spores of Aspergillus flavus (U.I. 81) to gamma irradiation before inoculation on Czapeks-Dox medium was investigated. The highest yielded in total aflatoxins was obtained with spores irradiated at 62.5 and 125.0 Krad while higher doses gave lower yields. Peak aflatoxin production occurred at 6 to 7 days of incubation with daily variations for both irradiated and non-irradiated cultures. However, fungal growth decreased with increased gamma irradiation dose. The effects of gamma irradiation on total aflatoxins (B1 + G1) produced by Aspergillus flavus (U.I . 81) growing on different foodstuffs and synthetic media have been investigated with particular regards to the consequences of pre-irradiation incubation period, irradiation medium and selected irradiation levels on the production of aflatoxins on these media. It was found that the media themselves, the pre-irradiation incubation period, and the gamma irradiation dose levels had profound effects on the total amounts of aflatoxins produced. The responses and aflatoxin productions of two strains of A. flavus; a toxigenic (U.I. 81) and a non-toxigenic strain (U.I. 39) to repeated doses of gamma irradiation were investigated over a period of 13 successive “generations”. Repeated exposure of the non-toxigenic strain of A. flavus to gamma irradiation did not induce aflatoxin production on Czapeks-Dox medium. However, the production of aflatoxin by the toxigenic strain depended on the dose level and the isolate. Generally, the irradiated spores produced less aflatoxins than the parent isolate or the non-irradiated sub-culture. Gamma irradiation induced mutation, the mutant being from the 3rd successive “generation” after repeated irradiation at 125.0 Krad. The characterization of the mutant was accomplished by (i) Cultural studies (ii) Toxin production, and (iii) Electron microscopic studies. Alterations in the structure of the cytoplasm occurred in the spores of the mutant. The effect of different doses of gamma irradiation on the macro-nutrients of a soya-gari diet inoculated with A. flavus was also investigated. Irradiation as high as 500.0 Krad did not cause any significant changes in the amino acids, total proteins, soluble carbohydrates, hemi-cellulose, cellolse, lignin and lipid contents. These results suggest that although the aflatoxins are fairly resistant to gamma irradiation, the control of Aspergillus flavus- contamination of foodstuffs, without any adverse effects on the organoleptic properties, can be achieved using gamma irradiation.