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Yeasts are unicellular microorganisms that belong to the kingdom of fungi. Yeasts are found in a diverse range of habitats, including soil, sugar-rich surfaces on the fruit and flowers of plants, and on the surface of plants and animals. Yeast plays an important role in soil by participating in nutrient cycles and mineralization processes. In contrast to the various studies investigating soil yeasts in Western countries, there is a dearth of research investigating soil yeasts in Africa and the Middle East. In these projects, we collected 493 soil samples from nine different geographical locations in Cameroon, and 562 soil samples from six geographical locations in Saudi Arabia. I identified yeast isolates by using the culture-based method and the fungal barcode sequence at the ITS region. In addition, the yeast species were analyzed using PCR- fingerprinting to investigate the genetic variation among strains within the same species. In sum, we have isolated and identified a total of 110 yeast isolates from Cameroon as well as a total of 114 yeast isolates from Saudi Arabia. Our results showed that soil samples from different regions in Cameroon had different yeast colonization (0%-56%), with the highest found in northwest Cameroon. The Saudi Arabian yeast isolation was different from those in Cameroon, with less diversity; the isolation rate ranged from 0.7 to 40.2 %, with the highest found in western Saudi Arabia. PCR-fingerprinting results suggested that yeast migration was common among the geographic regions within each country. However, local climatic conditions can significantly impact yeast diversity at the local and continental levels. / Thesis / Master of Science (MSc) / Yeasts are single-celled microorganisms that are widely dispersed in a wide variety of natural and human-related habitats such as plant leaves, flowers, fruits, water, and soil. Yeast is probably one of the earliest domesticated organisms; people have used yeast for fermentation and baking through much of the recorded human history. My project focused on studying yeast diversity in two geographically distinct countries, Cameroon and Saudi Arabia, in the context of environmental factors such as climate conditions and pH. I identified yeast present in soil samples morphologically, sequenced them at the ITS locus, and determined the genetic variation among isolates within individual yeast species using PCR fingerprinting. The analysis revealed differences in yeast species distribution and in yeast genotypic diversity among the analyzed geographic regions. These results suggest that Cameroon and Saudi Arabia contain significant novel yeast diversity.

Thesis (Ph. D.)--Auburn University, 2008. / Abstract. Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Missouri-Columbia, 2002. / Typescript. Vita. Includes bibliographical references (leaves 7191-205). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2002. / Typescript. Vita. Includes bibliographical references (leaves 7191-205). Also available on the Internet.

Thesis--Utrecht, 1935. / Summaries in Dutch, English and German. Includes bibliographical references (p. 21-24).

Yeast are exposed to many physical and chemical stresses when used in large-scale industrial fermentations, particularly the initial stages in which yeast are shifted from anaerobic storage to aerated wort. This work investigated the transcriptional and physiological responses of yeast that had been shifted from anaerobic to aerobic growth conditions. Microarray technology was employed to determine the transcriptional changes that occurred in the first hour of a pilot-plant fermentation compared to the 23rd hour. It was found that over 100 genes were up-regulated initially including genes involved in the synthesis of the essential membrane sterol ergosterol and genes for the protection of cells against oxidative stress. It was also determined that cells which accumulate ergosterol precursors in the absence of ergosterol were more sensitive to exogenous oxidative stresses, indicating a role for ergosterol in oxidative stress tolerance. Aeration of anaerobically grown cells did not affect their growth kinetics or viability. However, anaerobically grown cells were hypersensitive to exogenous oxidative stress compared to their aerobic counterparts. Anaerobic cells that underwent a short period of aeration prior to treatment with hydrogen peroxide generated a tolerance to the oxidant, indicating that the period of aeration produced an adaptive-like response. Microarray analysis of the cells during the period of aeration showed that representative genes from the oxidative stress response family were up-regulated rapidly and it was determined that the response was controlled by the Yap1p and Skn7p transcription factors. Deletion of the transcription factor genes indicated that they were responsible for the creation of tolerance to oxidant. Target gene products of the two transcription factors (Gpx2p, Gsh1p and Trx2p) were shown to be induced during the shift to aeration; however, the glutathione redox balance did not seem to be affected as the cells were shifted from highly reduced to oxidising environments. Unexpectedly, it was discovered that genes involved in the synthesis of amino acids were up-regulated during anaerobic growth and stringently downregulated upon aeration of cells. The transcriptional activator of those genes (Gcn4p) was essential for growth in anaerobic media which included amino acid supplementation.

Thesis--Københavns universitet. / "Translation by Niels Haislund"--T.p. verso. Includes bibliographical references (p. [128]-132).

Thesis--Københavns universitet. / "Translation by Niels Haislund"--T.p. verso. Includes bibliographical references (p. [128]-132).

Hybridization studies based on the prototrophic selection technique, involving the use of auxotrophic mutants of strains of all accepted species of the genus Kluyveromyces, are reported. Two main groups of mutually interfertile taxa were established within the genus. The first group comprises Kluyveromyces bulgaricus, Kluyveromyces cicerisporus, Kluyveromyces dobzhanskii, Kluyveromyces drosophilarum, Kluyveromyces fragilis, Kluyveromyces lactis, Kluyveromyces marxianus, Kluyveromyces phaseolosporus, Kluyveromyces vanudenii and Kluyveromyces wikenii. The second group consists of Kluyveromyces dabzhanskii, Kluyveromyces drosophilarum, Kluyveromyces laotis, Kluyveromyces vanudenii and Kluyveromyces wiokerhamii. Hybrids were also detected in crosses involving Kluyveromyces drosophilarum and Kluyveromyces waltii as well as Kluyveromyces marxianus and Kluyveromyces thermotolerans. In terms of the concept of the biological species and in compliance with the requirements of the International Code of Botanical Nomenclature, taxa which hybridize with Kluyveromyces marxianus and form fertile recombinants at frequencies observed in intraspecific crosses, are accepted as varieties of Kluyveromyces marxianus. Hybridization was observed between Kluyveromyces marxianus var. lactis and the presumed imperfect forms of some Kluyveromyces species, namely Candida kefyr, Candida macedoniensis and Torulopsis sphaerica. Recombination was not detected in crosses involving Kluyveromyces marxianus var. marxianus and representatives of other yeast genera, i.e. Pichia, Saccharomyces, Torulaspora and Zygosaccharomyces. Conclusions regarding the relationship between members of the genus Kluyveromyces, reached on the basis of this investigation are compared with those reported by other workers, who based their investigations on phenotypic characteristics as well as on the determinations of mol % G+C and DNA-DNA homology studies.