Ivor H. Evans

A novel colorimetric yeast bioassay for detecting trichothecene mycotoxins

A novel colorimetric microbial bioassay for toxicity has been developed; it shows particular sensitivity to trichothecene mycotoxins. The assay uses inhibition of expression of beta-galactosidase activity within the yeast Kluyveromyces marxianus as a sensitive toxicity indicator, cultures remaining yellow, rather than turning deep green-blue, in the presence of X-gal, a chromogenic substrate. The assay is conducted in standard microtitre plates, permitting small volumes (160 microl) and many replicates, and can be scored either automatically by a plate-reader, or by eye. Factors likely to affect the efficacy of the bioassay, including carbon source, solvents, inoculum cell density, and the use of membrane-modulating agents (MMAs), were assessed. Polymyxin B nonapeptide was the most effective toxicity-enhancing MMA tested, enabling the trichothecene mycotoxin, verrucarin A, to be detected at a concentration of about 1 ng/ml. The assays reproducibility was examined using polymyxin B sulfate, a cheaper MMA, and another trichothecene mycotoxin, T2 toxin: reproducibility and sensitivity were better for the beta-galactosidase X-gal endpoint than for an alternative chromogenic toxicity indicator, the respiratory substrate 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT).

A genetic analysis of glucoamylase activity in the diastatic yeast Saccharomyces cerevisiae NCYC 625

SummaryThe wild diastatic yeast Saccharomyces cerevisiae NCYC 625 has been shown to be homozygous for the glucoamylase-specifying gene STA2. spoII-1-mapping has positioned STA2 on chromosome II. Expression of STA2 is suppressed in some but not all diploids capable of sporulation, and is also inhibited by unlinked nuclear suppressor genes (SGL) found in some S. cerevisiae tester strains. EMS-induced glucoamylase-negative mutants often contain STA2-suppressor mutations. Depending on the allelic status of GEP1, a nuclear gene which also appears able to antagonise SGL-mediated suppression, STA2 expression can be blocked in petite mutants.

Transcriptomic and phenotypic analysis of the effects of T-2 toxin on Saccharomyces cerevisiae: evidence of mitochondrial involvement

At 5 μg mL(-1) , T-2 toxin significantly upregulated the transcription of 281 genes and downregulated 86. Strongly upregulated genes included those involved in redox activity, mitochondrial functions, the response to oxidative stress, and cytoplasmic rRNA transcription and processing. Highly repressed genes have roles in mitochondrial biogenesis, and the expression and stability of cytoplasmic rRNAs. T-2 toxin inhibition of growth was greater in a medium requiring respiration, and was antagonized by antioxidants. T-2 toxin treatment induced reactive oxygen species, caused nucleolytic damage to DNA, probably mitochondrial, and externalization of phosphatidylserine. Deletion mutations causing respiratory deficiency substantially increased toxin tolerance, and deletion of some TOR (target of rapamycin) pathway genes altered T-2 toxin sensitivity. Deletion of FMS1, which plays an indirect role in cytoplasmic protein synthesis, markedly increased toxin tolerance. Overall, the findings suggest that T-2 toxin targets mitochondria, generating oxy-radicals and repressing mitochondrial biogenesis genes, thus inducing oxidative stress and redox enzyme genes, and triggering changes associated with apoptosis. The large transcriptional changes in genes needed for rRNA transcription and expression and the effects of deletion of FMS1 are also consistent with T-2 toxin damage to the cytoplasmic translational mechanism, although it is unclear how this relates to the mitochondrial effects.

Uptake of aflatoxin B1 and T-2 toxin by two mycotoxin bioassay microorganisms: Kluyveromyces marxianus and Bacillus megaterium.

Abstract. Uptake of aflatoxin B1 (AFB1) and trichothecene T-2 toxin from growth medium by mycotoxin bioassay strains of Kluyveromyces marxianus and Bacillus megaterium was assessed by incubating, washing, and sonicating the cells, extracting samples with chloroform, and analysing the extracts by a combination of high-performance thin-layer chromatography (HPTLC) and fluorescence densitometry. Using cultures of K. marxianus, the entire AFB1 dose was recovered and no AFB1 metabolites were detected. Less than 1% of the AFB1 was recovered from the cells, and AFB1 did not inhibit growth. Methanol in the incubation medium had no significant effect on the levels of AFB1 associated with K. marxianus cells. The entire dose of T-2 toxin was also recovered from K. marxianus cultures, and no metabolites were detected; again, less than 1% of T-2 toxin was cell-associated, but growth was completely inhibited. AFB1 partially inhibited the growth of B. megaterium; approximately 12% of the dose could not be recovered, and no AFB1-related metabolites were detected. Methanol increased the levels of recoverable AFB1 associated with B. megaterium cells. In the case of T-2 toxin, around 8% of the dose was not recovered, and no metabolites were detected; growth of B. megaterium was stimulated. These results suggest irreversible binding of both toxins, or derivatives of them, to the cells of B. megaterium.

A sensitive bioassay for the mycotoxin aflatoxin B1, which also responds to the mycotoxins aflatoxin G1 and T-2 toxin, using engineered baker's yeast

A novel aflatoxin B(1) bioassay was created by introducing a Lipomyces kononenkoae alpha-amylase gene into a strain of S. cerevisiae capable of expressing the human cytochrome P450 3A4 (CYP3A4), and the cognate human CYP450 reductase. This strain and a dextranase-expressing strain were used in the development of a microtitre plate mycotoxin bioassay, which employed methanol as the solvent and polymyxin B nonapeptide as a permeation enhancer. Stable co-expression of the CYP3A4 gene system and of the dextranase and amylase genes in the two bioassay strains was demonstrated. The bioassay signalled toxicity as inhibition of secreted carbohydrase activity, using sensitive fluorimetric assays. The amylase-expressing strain could detect aflatoxin B(1) at 2 ng/ml, and was more sensitive than the dextranase-expressing strain. Aflatoxin G(1) could be detected at 2 microg/ml, and the trichothecene mycotoxin T-2 toxin was detectable at 100 ng/ml.

Isolation of Mitochondrial DNA

6.) Homogenate will need to be filtered prior to centrifugation. Two filtrations are required. First, using a 4L beaker, suspend 2 layers of sterilized cheesecloth (with enough extra to use to squeeze excess liquid). Pour homogenate SLOWLY over cheesecloth and allow to filter through. When liquid stops dripping from cheesecloth, while wearing gloves, squeeze out extra solution. Keeping filtrate and adding an additional 250-500mls of Buffer A and re-homogenize. Repeat the filtering process, but this time use 2 layers of MIRACLOTH, (note be careful when squeezing... MIRACLOTH has low tensile strength and you if you use too much POWER :) your filtrate will cover your lab bench... you have been warned!)

Electrophoretic karyotype of the amylolytic yeast Lipomyces starkeyi and cloning, sequencing and chromosomal localization of its TRP1 gene

Abstract The genome of the amylolytic yeast strain Lipomyces starkeyi NCYC 1436 was analysed using contour-clamped homogeneous electric field gel electrophoresis (CHEF). The banding pattern under a variety of running conditions indicating the presence of 11 different chromosome-sized DNA molecules. The sizes of these chromosome bands were determined by comparison with chromosomes from standard strains of Schizosaccharomyces pombe and Saccharomyces cerevisiae. The chromosomal bands were estimated to be within the range 0.7–2.8 Mb, with the genome (excluding mitochondrial DNA) estimated at 15 Mb. The molecular cloning of the TRP1 gene, isolated from a genomic library of this strain, is also reported: the gene was present on a 6.5-kb Sau3A DNA fragment, and complemented the trpC gene of E. coli. The DNA sequence was determined (EMBL accession No. Z68292) and compared to other tryptophan biosynthetic genes encoding N-(5′-phosphoribosyl) anthranilate isomerase (PRAI) activity. The gene was also used as a probe in hybridization studies, and by this means, its chromosomal location was identified.

Sodium phosphate enhancement of starch hydrolysis by a diastatic strain of Saccharomyces cerevisiae

SummaryLow concentrations of inorganic phosphate stimulate by up to 2-fold aerobic growth ofS. cerevisiae N.C.Y.C. 625 in starch medium. No such stimulation of growth is seen in a glucose medium. Enhanced growth on starch appears to result from increased secretion of glucoamylase (1,4-α-D-glucan glucohydrolase, EC 3.2.1.3) into the culture medium.