Degradation of aflatoxin by Aspergillus flavus

Aflatoxin degradative activity was demonstrated in 6- to 12-d-old intact mycelium and cell-free extracts of Aspergillus flavus. The addition of cycloheximide, SKF 525-A or metyrapone to cultures of A. flavus prevented subsequent degradation of the aflatoxins, while in cell-free extracts degradation was inhibited by SKF 525-A, metyrapone and cytochrome c but not by KCN. In cell-free extracts, aflatoxin degradation was enhanced by NADPH and NaIO4. The results suggest the involvement of cytochrome P-450 monooxygenases in the aflatoxin degradative activity of A. flavus.     

The effect of eucalyptus oil on growth and aflatoxin production by Aspergillus flavus

The effect of eucalyptus oil on growth and aflatoxin production by Aspergillus flavus was tested at three levels, viz . 0·05, 0·1 and 0·2 ml/50 ml SMKY medium. After 6 days of incubation on 0·05 and 0·1 ml supplemented SMKY medium, growth and toxin production were inhibited while at 0·2 ml concentration there was no growth. However, after 12 days of incubation toxin production was greater than the controls.     

Viral influences on aflatoxin formation by Aspergillus flavus

Summary A nontoxigenic isolate of Aspergillus flavus (NRRL 5565) contains a viral genome consisting of 3 double-stranded RNA (ds-RNA) components with molecular weights of approximately 3 kb each. It thus shares a characteristical feature with a virus occuring in strains of Penicillium chrysogenum.Application of known inhibitors of doublestranded RNA virus synthesis results in stable aflatoxin formation by this originally nontoxigenic strain and the simultaneous loss of its ds-RNA traits. Since the inhibitor induced toxicity can be completely reverted by incubation with a virus from Penicillium chrysogenum (PcV), it is presumed that PcV or a functional related virus possibly constitutes the aflatoxin repressing determinant in Aspergillus flavus.     

5-Azacytidine inhibits aflatoxin biosynthesis in Aspergillus flavus

Aflatoxins, mainly produced by Aspergillus flavus and A. parasiticus, are a group of potent mycotoxins with carcinogenic, hepatotoxic, and immunosuppressive properties. Many studies have been devoted to investigating their biosynthesis mechanism since they were discovered half a century ago. 5-Azacytidine (5-AC), a derivative of the nucleoside cytidine and an inactivator of DNA methyltransferase, is widely used for studies in epigenetics and cancer biology, and has also been used for studying secondary metabolism in fungi. In this study, 5-AC was applied to investigate its effect on the development and aflatoxin biosynthesis of A. flavus. The results indicate that 5-AC inhibits the ability to produce aflatoxin and also causes a fluffy aconidial phenotype. Further studies revealed that 5-AC affects gene expression of A. flavus to a limited degree, and the unique homolog of DNA methyltransferase gene (DmtA) expressed constitutively during different developmental stages of A. flavus irrespective of 5-AC. This work may provide some basic data to elucidate the role of 5-AC in aflatoxin biosynthesis and the development of A. flavus.     

Influence of woodsmoke on aflatoxin production by Aspergillus flavus

Summary Woodsmoke delayed aflatoxius B₁ and G₁ release and significantly exerted inhibitory effects on the toxins production by a toxigenic Asperigillus flavus. The fungistatic efficiency of the woodsmoke increased with reduced moisture content in fish.     

黑曲霉对黄曲霉生长、产毒及黄曲霉毒素B1的影响

目的研究黑曲霉对黄曲霉生长、产毒的抑制作用及对AFB1的降解作用。方法将黑曲霉分别与黄曲霉、AFB1共同培养,定期测定培养液pH、菌丝体干重、黄曲霉孢子数、AFB1含量。结果黑曲霉与黄曲霉混合培养时,黄曲霉孢子数、AFB1含量均比单独培养的低,2组之间差异有统计学意义(P<0.05),抑制率达到68.06%~91.52%;加入黑曲霉后,AFB1含量降低,实验组与对照组之间差异有统计学意义(P<0.05),降解率为46.19%。结论黑曲霉既能抑制黄曲霉生长、产毒,又能降解AFB1。     

Genetics of Aspergillus flavus: linkage of aflatoxin mutants

Eight aflatoxin (afl) mutants of Aspergillus flavus were induced with N-methyl-N'-nitro-N-nitrosoguanidine. Heterozygous diploids formed between afl mutants and tester strains revealed that each afl mutant was recessive. Haploids selected from these heterozygous diploids indicated the linkage of all eight afl mutants to markers on group VII. These include previously mapped arg-7 (arginine), leu (leucine), dominant afl-1, and nor which accumulates norsolorinic acid that is visible as an orange-red pigment. Diploid complementation tests indicated that all but two afl mutants were nonallelic. Diploids homozygous for nor, resulting from crossing-over, were isolated and used to map new afl genes.     

Effect of phytate on aflatoxin formation by Aspergillus parasiticus and Aspergillus flavus in synthetic media

The effect of phytate on the production of aflatoxins by Aspergillus parasiticus and Aspergillus flavus grown on synthetic media was examined. In the absence of pH control (initial pH 4.5–6.5) for A. parasiticus, phytate (14.3 mM) caused a six-fold decrease in aflatoxins in the medium and a ten-fold decrease in those retained by the mycelia. When the initial pH of the medium was adjusted to 4.5 no effect on aflatoxin production was observed. With A. flavus or A. parasiticus grown on media with a higher initial pH value (6 to 7), the presence of phytate in the media caused an increase in aflatoxin production. These results are inconsistent with previous studies which indicated that phytate depresses aflatoxin production by rendering zinc, a necessary co-factor for aflatoxin biosynthesis, unavailable to the mold.     

Interaction between Streptococcus lactis and Aspergillus flavus on production of aflatoxin

The inoculation of Aspergillus flavus spores into a culture of Streptococcus lactis in Lablemco tryptone broth medium resulted in little or no aflatoxin accumulation even though the growth of the fungus was not hindered. The drop in pH and reduced nutrient levels in the medium as a result of the S. lactis growth were not the cause of the observed inhibition. The inhibition was not eliminated by the addition of carbohydrate equal to the amount used by the bacterium before the inoculation with the fungus. Aflatoxin levels were also markedly reduced when S. lactis was inoculated into a growing A. flavus culture. In addition to inhibiting the synthesis of aflatoxin, S. lactis also degraded preformed toxin. A. flavus, on the other hand, not only reduced the growth of S. lactis but also affected the morphology of the bacterial cell; the cells became elongated and formed long chains. S. lactis produced and excreted the inhibitor into the medium late in its growth phase. The inhibitor was a heat-stable low-molecular-weight compound. Chloroform extracts of A. flavus grown in the presence of S. lactis were toxic to Bacillus megaterium but did not exhibit mutagenic or carcinogenic activity in the Salmonella/mammalian microsome mutagenicity test.     

Substrate-induced lipase gene expression and aflatoxin production in Aspergillus parasiticus and Aspergillus flavus

AIMS: To establish a relationship between lipase gene expression and aflatoxin production by cloning the lipA gene and studying its expression pattern in several aflatoxigenic and nontoxigenic isolates of Aspergillus flavus and A. parasiticus. METHODS AND RESULTS: We have cloned a gene, lipA, that encodes a lipase involved in the breakdown of lipids from aflatoxin-producing A. flavus, A. parasiticus and two nonaflatoxigenic A. flavus isolates, wool-1 and wool-2. The lipA gene was transcribed under diverse media conditions, however, no mature mRNA was detected unless the growth medium was supplemented with 0.5% soya bean or peanut oil or the fungus was grown in lipid-rich medium such as coconut medium. The expression of the lipase gene (mature mRNA) under substrate-induced conditions correlated well with aflatoxin production in aflatoxigenic species A. flavus (SRRC 1007) and A. parasiticus (SRRC 143). CONCLUSIONS: Substrate-induced lipase gene expression might be indirectly related to aflatoxin formation by providing the basic building block 'acetate' for aflatoxin synthesis. No direct relationship between lipid metabolism and aflatoxin production can be ascertained, however, lipase gene expression correlates well with aflatoxin formation. SIGNIFICANCE AND IMPACT OF THE STUDY: Lipid substrate induces and promotes aflatoxin formation. It gives insight into genetic and biochemical aspects of aflatoxin formation.     

Effect of temperature on production of aflatoxin on rice by Aspergillus flavus

Summary The effect of temperature on formation of aflatoxin on solid substrate (rice) byAspergillus flavus NRRL 2999 has been studied in some detail. The optimum temperature for production of both aflatoxin B₁ and G₁ under the conditions employed is 28° C. Comparable yields of B₁ were obtained at 32° C, but considerably less G₁ was produced at this temperature. Both B₁ and G₁ were found in lesser amounts at temperatures above 32° C, and the aflatoxin content of rice incubated at 37° C was low (300–700 ppb) even though growth was good.Reducing the temperature from 28° to 15° C resulted in progressively less aflatoxin, but 100 ppb of B₁ was detected in cultures incubated 3 weeks at 11° C. No aflatoxin was produced at 8° C.The ratio of the four aflatoxins is affected by temperature. At the lower temperatures, essentially equal amounts of aflatoxin B₁ and G₁ were produced, whereas at 28° C, approximately four times as much B₁ was detected as G₁. At the higher temperatures, relatively less G was formed, until at 37° C, less than 10 ppb was detected.This is a laboratory of the Northern Utilization Research and Development Division, Agricultural Research Service, U.S. Department of Agriculture.     

Structural modification of cuminaldehyde thiosemicarbazone increases inhibition specificity toward aflatoxin biosynthesis and sclerotia development in Aspergillus flavus

Applied Microbiology and Biotechnology - Aspergillus flavus is an opportunistic mold that represents a serious threat for human and animal health due to its ability to synthesize and release, on...     

Identification of genes differentially expressed during aflatoxin biosynthesis in Aspergillus flavus and Aspergillus parasiticus

A complex regulatory network governs the biosynthesis of aflatoxin. While several genes involved in aflatoxin production are known, their action alone cannot account for its regulation. Arrays of clones from an Aspergillus flavus cDNA library and glass slide microarrays of ESTs were screened to identify additional genes. An initial screen of the cDNA clone arrays lead to the identification of 753 unique ESTs. Many showed sequence similarity to known metabolic and regulatory genes; however, no function could be ascribed to over 50% of the ESTs. Gene expression analysis of Aspergillus parasiticus grown under conditions conducive and non-conductive for aflatoxin production was evaluated using glass slide microarrays containing the 753 ESTs. Twenty-four genes were more highly expressed during aflatoxin biosynthesis and 18 genes were more highly expressed prior to aflatoxin biosynthesis. No predicted function could be ascribed to 18 of the 24 genes whose elevated expression was associated with aflatoxin biosynthesis.