Enhanced Aflatoxin Production by Aspergillus flavus and Aspergillus parasiticus after Gamma Irradiation of the Spore Inoculum

Abstract

Distilled water plus 0.1% surfactant suspensions of spores of Aspergillus flavus and Aspergillus parasiticus were exposed to several radiation levels of cobalt-60 gamma rays. Spores of A. flavus isolate M-141 were exposed to radiation levels of approximately 16, 90 and 475 Krads and inoculated onto a sterile rice substrate which was then monitored for aflatoxin production. In this initial trial with A. flavus M-141, aflatoxins B1 and M production on rice increased as radiation dose increased. At the highest dose, this increase was more than 50 times higher than the non-irradiated controls. Spores of an aflatoxin G1-producing A. parasiticus isolate, M-1094, were exposed to 90, 215 and 430 Krads and resulted in increased production of aflatoxins G1, B1, and M. Aflatoxin production by M-1094 was highest at the low and medium dose levels. Irradiation of spores of this isolate with 430 Krads produced no observable spore germination or growth on rice and no detectable aflatoxin after 1 week of incubation at 27 C. A typical colonies from irradiated spores were selected and their mycotoxin production was determined. Increase in aflatoxin production by these strains, as compared to non-irradiated controls, was 67:1 for aflatoxin B1, 136:1 for B2, and 138:1 for M. This potential for greatly increased mycotoxin production must be considered when food is irradiated or when a high production of aflatoxins is desired.

In vitro Selection of Maize Rhizobacteria to Study Potential Biological Control of Aspergillus Section Flavi and Aflatoxin Production

“…Cotty and Bhatnagar (1994) showed that one atoxigenic A. flavus strain which produced many of the enzymatic activities present in the aflatoxin biosynthesis pathway but did not produce aflatoxins, was the most effective atoxigenic strain for reducing contamination in greenhouse tests in cotton bolls. The problem of the use of non-toxigenic strains of A. flavus for out-competing toxigenic strains, is that certain atoxigenic strains of A. flavus are known to be unstable and can convert to a highly toxigenic phenotype (Schindler et al, 1980;Clevstrom and Ljunggren, 1985). The stability of the non-aflatoxin producing phenotype may be an important consideration in selecting strains for use in strategies to prevent aflatoxin contamination through intraspecific competition (Cole and Cotty, 1990;Brown et al, 1991).…”

Irradiation for Mold and Mycotoxin Control: A Review

“…A similar effect was observed by other researchers. Irradiated strains of A. flavus , A. parasiticus , A. niger , and A. ochraceus also produced more AFB 1 or OTA and then nonirradiated strains (Schindler and others ; Ribeiro and others ). However, this finding is not a consensus observation because other researchers reported the opposite response; for example, irradiated spores of A. parasiticus did not produce more AFB 1 than nonirradiated spores on rice (Sharma and others ).…”

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“…On the other hand, it was noticed that the survival of A. flavus on FM medium was significantly (P a 0.05) higher than on DM medium and there were significant differences between irradiation treatments and storage time (P a 0.05). On the other hand, several subsequent studies [29][30][31] showed the enhancement of mycotoxin production resulting from c-irradiation. There are a number of reports which suggest that moulds are very sensitive to c-radiation and that a dose of 1.0-4.0 kGy eliminated viable fungi [7,14,24].…”