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

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.

Temperature Limits for Production of Aflatoxin by Twenty-five Isolates of Aspergillus flavus and Aspergillus parasiticus.

Aflatoxicogenic isolates of Aspergillus were tested for their aflatoxin production after 8 weeks of growth on wort agar medium at high (41, 46 ± 1 C) and low (2, 7 ± 0.5 C) temperatures. Controls were grown at approximately 22 C, a temperature known to be favorable for aflatoxin production. There were two replications of each treatment. All replicate cultures of the 25 isolates grown at 22 C were positive for aflatoxin. Aflatoxins were not detected in wort agar when cultures were incubated at other test temperatures. It appears that both Aspergillus flavus and Aspergillus parasiticus will not produce aflatoxins when grown at ⩽ 7.5 C or at ⩾ 40 C.

Aflatoxin Production by Aspergillus flavus as Related to Various Temperatures.

Two aflatoxin-producing isolates of Aspergillus flavus were grown for 5 days on Wort media at 2, 7, 13, 18, 24, 29, 35, 41, 46, and 52 C. Maximal production of aflatoxins occurred at 24 C. Maximal growth of A. flavus isolates occurred at 29 and 35 C. The ratio of the production of aflatoxin B(1) to aflatoxin G(1) varied with temperature. Aflatoxin production was not related to growth rate of A. flavus; one isolate at 41 C, at almost maximal growth of A. flavus, produced no aflatoxins. At 5 days, no aflatoxins were produced at temperatures lower than 18 C or higher than 35 C. Color of CHCl(3) extracts appeared to be directly correlated with aflatoxin concentrations. A. flavus isolates grown at 2, 7, and 41 C for 12 weeks produced no aflatoxins. At 13 C, both isolates produced aflatoxins in 3 weeks, and one isolate produced increasing amounts with time. The second isolate produced increasing amounts through 6 weeks, but at 12 weeks smaller amounts of aflatoxins were recovered than at 6 weeks.