C.H. Pohl

A novel oxylipin-associated 'ghosting' phenomenon in yeast flocculation.

Research on the distribution of oxylipins (3-hydroxy fatty acids) in flocculant strains of the yeast Saccharomyces cerevisiae led to the uncovering of a novel ‘ghosting’ phenomenon observed during assumed lectin-mediated aggregation. We found that intracellular oxylipin-containing osmiophilic layers migrate through yeast cell walls in a ‘ghostlike’ fashion without visually affecting the cell wall structure or the layers. This migration resulted in the binding of these layers to cell walls of adjacent cells. Consequently, ‘ghosting’ seems a prerequisite for flocculation to occur. However, ‘ghosting’ alone may not be sufficient to ensure flocculation.

Oxylipins and ascospore morphology in the ascomycetous yeast genus Dipodascus

Immunofluorescence microscopy was used to assess members of the yeast genus Dipodascus for the presence of 3-hydroxy oxylipins. Fluorescence was associated with the aggregating ascospores in all species tested, thus suggesting the association of 3-hydroxy oxylipins with these cells, especially the surrounding slime sheaths. An ultrastructural study of the ascospores revealed sheaths with indentations, probably caused by the close packing of the ascospores to form clusters. In addition, an increase in the neutral and glycolipid fractions as well as a decrease in the phospholipid fraction during ascosporogenesis in D. ambrosiae was found.

Bioprospecting for novel hydroxyoxylipins in fungi: presence of 3-hydroxy palmitic acid in Saccharomycopsis malanga

Electron microscopy studies indicated that the major oxylipin 3-hydroxy palmitic acid (16:0) was associated with aggregating vegetative cells and formed a web-like structure around these cells. Cross sections through this structure showed a hydrophilic outer layer and a more hydrophobic inner layer suggesting that the web-like structure is in fact tube-like micelles. This information sheds more light on the role of these hydroxyoxylipins in fungi.

The production of γ-linolenic acid by selected members of the Dikaryomycota grown on different carbon sources

In this study, seven fungal strains, representing different phylogenetic groups within the Dikaryomycota, were tested for the presence of γ-linolenic acid [18:3(ω6)], when grown in synthetic liquid media devoid of fatty acids, on a series of 40 different carbon sources. The fungal strains represented the species Dipodascopsis uninucleata, Eurotium rubrum, Galactomyces geotrichum, Neurospora crassa, Saccharomyces cerevisiae, Spongipellis unicolor and Talaromyces flavus. Cultures were periodically harvested during growth and the fatty acids in the total lipids analysed as methyl esters, using gas chromatography and mass spectrometry. It was found that 18:3(ω6) is present in E. rubrum CBS 350.65, S. unicolor CBS 117.16 and in T. flavus CBS 310.38NT, when these strains were grown on certain carbon sources. No correlation between the growth phase of the organism and the presence of 18:3(ω6) could be detected. In order to confirm the production of 18:3(ω6), the lipid metabolism of two unrelated dikaryomycotan fungi (S. unicolor CBS 117.16 and E. rubrum CBS 350.65) grown on two different carbon sources each, was examined. Cultures of E. rubrum CBS 350.65 were grown on glucose and sorbose and cultures of S. unicolor CBS 117.16 on glucose and sucrose in synthetic liquid media with a C:N ratio of 50:1 (w/w). The total lipids of these cultures were fractionated and the fatty acids in the fractions analysed as methyl esters, using gas chromatography and mass spectrometry. The lipid metabolism of both E. rubrum CBS 350.65 and S. unicolor CBS 117.16 differed on the two carbon sources used. The ab initio production of 18:3(ω6) by E. rubrum CBS 350.65 in synthetic liquid media was confirmed. In contrast, the ab initio production of 18:3(ω6) by S. unicolor CBS 117.16 in synthetic liquid media could not be confirmed.

Characterisation of yeasts isolated from traditional opaque beer beverages brewed in Zimbabwean households

Variability exists in raw materials and processing methods used to produce household traditional opaque beers in Zimbabwe, resulting in beers of variable quality, depending on fermenting micro-organisms involved. Yeasts are important in determining the alcohol content, nutrition and organoleptic properties of the beers. This study aimed at determining the diversity and characteristics of the predominant yeasts isolated from a variety of beers collected from rural households in different geographical localizations. Predominant yeasts from 13 beer samples were characterized using morphological, biochemical and physiological tests. A total of 14 morphologically different yeasts were isolated. Yeast counts in the beer samples ranged from 7.87 to 9.56 log colony forming units/ml. From the 14 yeast isolates, a total of 11 yeasts were identified to species level. Saccharomyces cerevisiae was the predominant species identified in the beers. Other yeast species identified in the beers were Issatchenkia occidentalis, Kluyveromyces marxianus, Candida glabrata and Sporobolomyces holsaticus. Two yeast isolates were identified as belonging to the genus Rhodotorula. Ten of the isolates were able to ferment at least one of the fermentation substrates D-glucose, D-galactose, maltose, sucrose and raffinose, while three isolates were incapable of fermenting any of the fermentation substrates used. None of the isolates were able to ferment lactose. Five of the S. cerevisiae isolates were able to grow at 40°C while K. marxianus was the only isolate capable of growing at 45°C. Key words: Yeasts, characterisation, traditional opaque beer.

Variation in functional ascospore parts in the ascomycetous yeast Dipodascopsis uninucleata

A variation in functional ascospore morphology was detected using electron microscopy (EM) in two varieties of the yeast Dipodascopsis uninucleata, i.e., D. uninucleata var. uninucleata and D. uninucleata var. wickerhamii. It was found that the latter produces ascospores characterized by the absence of small surface hooks which have been implicated in the release and re-assembly of ascospores in D. uninucleata var. uninucleata. These varieties are closely related on the basis of their mode of sexual reproduction, ascospore morphology as observed under the light microscope, physiological characteristics as well as the extent of divergence in the variable D1/D2 domain of the large subunit 26S ribosomal DNA.

Uncovering the first double brimmed hat-shaped ascospores in Ambrosiozyma platypodis Van der Walt

Using transmission electron microscopy with glutaraldehyde and osmium tetroxide as chemical fixatives, hatshaped ascospores with two brims each were uncovered in the yeast Ambrosiozyma platypodis. This is the first report on such structures.