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.

Há evidência de interação mutualista entre leveduras e abelhas sem ferrão, em que estas são vetores de leveduras que servem de alimento e modificam produtos do ninho das abelhas. Vários estudos registraram a ocorrência de espécies dos gêneros Candida e Starmerella associadas ao ninho de diferentes meliponíneos. Nesse estudo foram identificadas as espécies de leveduras vetorizadas por Frieseomellita varia, Scaptorigona polysticta, Sacaptorigona postica, Tetragonisca angustula angustula, Melipona compressipes manaoensis e Melipona scutellaris em regiões e meliponários no Cerrado tocantinense, e feita uma análise das características funcionais das leveduras vetorizadas pela abelha Scaptorigona postica. Amostragens foram realizadas entre os anos de 2006 a 2015 em sistemas de meliponicultura localizados em quatro municípios do Estado do Tocantins, região norte do Brasil, sendo espécimes de abelhas campeiras capturadas na entrada dos ninhos com sacos plásticos e postas para caminhar em placas contendo meio YMA. Os isolados foram identificados por métodos moleculares, realizando-se extração de DNA, agrupamento por reação em cadeia da polimerase (PCR) utilizando o primer EI-1 e sequenciamento do domínio D1/D2 do gene 26S rDNA utilizando os primers NL-1 e NL-4. As características funcionais das leveduras vetorizadas por S. postica foram mensuradas por testes fisiológicos e bioquímicos para as habilidades de assimilar oito compostos de carbono e quatro álcoois, fermentação de glicose, crescimento em meio contendo ácido acético, meio osmofílico e nas temperaturas 37°C e 40°C. Entre os diferentes meliponineos, foram identificados 62 espécies de leveduras, classificadas em 18 gêneros do filo Ascomycota e seis do filo Basidiomycota, sendo observado uma maior frequência das espécies Torulaspora delbrueckii, Candida apicola, Pichia membranifaciens, Pichia kluyveri e Starmerella meliponinorum, representando 41% do total de isolados. Destas, C. apicola e S. meliponinorum são frequentes em meliponineos, enquanto que as outras são típicas de substratos açucarados, mostrando que as abelhas vetorizam leveduras de seus ambientes de coleta de polén e néctar. Diferenças na composição de espécies ocorreram entre Meliponini e Trigonini, mostrando estratégias diferentes de visitação nos substratos contendo leveduras. Ainda, cada abelha vetoriza uma comunidade típica de leveduras, o que pode indicar hábitos seletivos e diferenciais de coleta. A análise das características funcionais de 52 linhagens isoladas de S. postica revelou que em grande parte das leveduras assimilam os carboidratos galactose, maltose, celobiose e trealose, em quanto que o número de leveduras que assimilam sacarose, amido, melibiose e xilose foi menor. O etanol foi assimilado por 36,53% das leveduras, mas acetona, metanol e isopropanol foram pouco frequentes. Grande parte das leveduras foi fermentadora, osmofilica e resistente ao ácido acético, mostrando adaptação a altas concentrações de açucares e ambiente fermentativo. / There are evidences for a mutualistic interaction between yeasts and stingless bees, in which bees vector yeasts that serve as food item or modify the products of the nest. Various studies report the occurrence of species of the generea Candida e Starmerella associated with nests of different meliponini. In this study we identified the yeast species vectored by Frieseomellita varia, Scaptorigona polysticta, Sacaptorigona postica, Tetragonisca angustula angustula, Melipona compressipes manaoensis and Melipona scutellaris in areas of Cerrado, and we made a funtional analysis of the yeasts associated with Scaptorigona postica. Sampling was done between the years 2006 and 2015 in Tocantins State, Northern Brasil, and the collector bees were captured iin the nest entrance with sterile bags and put to walk on agar platescontaining YMA. Yeast isolates were identified by molecular methods using DNA extraction, PCR fingerprinting with primer EI-1 and sequencing of D1/D2 region of the gene 26S rDNA uing primers NL-1 and NL-4 of representative strains. Functional characters were tested as carbohydrate assimilation, osmotolerance and acid tolerance, and resistance to 37°C and 40°C for yeasts isolated from Scaptorigona postica. Among all stingless bees, 62 yeast species belonging to 18 genera of Ascomycota and six Basidiomycota were identified, with a higher frequency ofTorulaspora delbrueckii, Candida apicola, Pichia membranifaciens, Pichia kluyveri and Starmerella meliponinorum that represented 41% of the strains. Among these, C. apicola and S. meliponinorum are frequent in stingless bees, and the others are commonly isolated from sugary substrates, indicating that the bees vector yeasts from the enviroments where they collect nectar and pollen. Significant differences in the yeast composition occur between Meliponini and trigonini, indicating different strategies of visitation in the substrates colonized by yeasts. Also each bee species carries different yeast communities, indicating a selective and different collecting habit. Functional characters of 52 yeasts isolated from S. postica revealed that most assimilate simple sugars galactose, maltose, celobiose, and trealose, bu only ¼ of them assimilate sucrose and xylosis. Assimilation of starch and mellibiose was uncommon. Ethanol was assimilated by 36,53% of the yeasts but acetona, metanol and isopropanol were less frequently assimilated. Most yeasts was fermentative, osmophillic and resistant to acetic acid which indicates an adaptation to high sugar concentrations and the fermentative environment.

<p><strong><br /></strong></p> / <p><strong>In Canada the incidence of yeast infections have increased over the past decade, which in turn has resulted in the increased mortality and morbidity rates among the immuno-compromised patients. Yeasts are ubiquitous in nature and constitute a healthy portion of human skin and gut flora. Factors such as the urban settings and food have been previous shown to influence the yeast flora people harbour. This makes us believe that to effectively tackle the rising yeast infections in Canada we need to not only conduct epidemiological yeast studies in clinical settings but should also understand the diversity and distribution of them in the urban environment. This thesis constitutes of an epidemiological fungemia study and an urban environmental yeast profiling study conducted in the city of Hamilton.</strong></p> <p><strong> </strong></p> <p><strong>In the first chapter of the thesis I discuss the results of the epidemiological candidemia study. We noticed that over the past decade the mean age of the population with candidemia in hospitals within Hamilton has increased by 10 years. DNA fingerprinting analysis suggested that 33% of the blood stream</strong><strong><em>Candida</em></strong><strong> </strong><strong>isolates from January 2005 to February 2009 belonged to 18 clusters, some of which were shared between wards and hospitals. we found that for each of the four species, strains isolated closer to each other temporally were overall genetically more similar to each other as well, which suggested that nosocomial sources likely caused repeated candidemia infections. The study is the first of its sort in Canada and the results of this chapter are expected to aid infection control practitioners in the Hamilton hospitals and make the stay of patients in hospitals safer.</strong></p> <p><strong> </strong></p> <p><strong>In the second chapter, we discuss the diversity and distribution of yeasts prevalent on trees in and around Hamilton. We identified a total of 88 environmental yeasts belonging to 20 species (based on ITS sequence data). The yeast populations were highly heterogeneous in both species and genotype composition. Among the 14 tree species sampled, yeasts were frequently found on cedar, cottonwood and basswood. Interestingly all the</strong><strong> <em>Candida parapsilosis</em> </strong><strong>strains were found from pine tree only. Some of the potential environmental factors shaping the distribution of yeast populations in Hamilton are discussed.</strong></p> / Master of Science (MSc)