Browsing by Author "Tekolo, Obakeng McDonald"
Now showing 1 - 1 of 1
Results Per Page
- ItemThe osmotic stress response of basidiomycetous yeasts(Stellenbosch : Stellenbosch University, 2007-12) Tekolo, Obakeng McDonald; Prior, B. A.; Botha, Alfred; Stellenbosch University. Faculty of Science. Dept. of Microbiology.ENGLISH ABSTRACT: Basidiomycetous yeasts are found in a wide range of geographical areas ranging from tropical forests to desert regions. These yeasts are associated with different habitats such as soil, decaying vegetative debris, living plants and animals. Some may even be opportunistic human pathogens. In most of these habitats the yeasts may periodically be exposed to adverse conditions such as osmotic stress. Forty-one basidiomycetous yeast strains obtained from culture collections and isolated from nature were studied using various methods which includes the determination of different minimum water activities (aw; NaCl or sorbitol) for growth, survival in soil of varying moisture content, intracellular osmolytes accumulated and their release upon hypo-osmotic stress. The growth of most strains showed greater tolerance to NaCl than sorbitol at the same level of water activity. Interestingly, there were no basidiomycetous strains that showed growth below 0.90aw. 13C nuclear magnetic resonance (NMR) spectroscopy and high performance liquid chromatography (HPLC) was used to analyze the osmolytes accumulated by all the strains of basidiomycetous yeasts when grown at 0.95 aw (NaCl). Glycerol was the major solute accumulated intracellulary by all the yeasts. Arabitol, mannitol or trehalose was accumulated in addition to glycerol in most yeasts whereas a number of yeasts only accumulated glycerol when grown at reduced aw. However, Cryptococcus laurentii US 1F was an exception by accumulating three solutes intracellularly when grown at reduced water activity. When exposed to hypo-osmotic shock all three solutes were rapidly released from the cells. Cryptococcus hungaricus CBS 5421, Cryptococcus macerans CBS 2206 and Cryptococcus neoformans US I1 were further analyzed with 13C NMR spectroscopy to determine whether the type of osmolyte accumulated during different phases of growth at 0.95aw (NaCl) might change. No changes were observed as the same osmolytes were accumulated in all cases. Five yeast strains (C. neoformans US I1, Rhodotorula mucillaginosa CBS 5951, C. macerans CBS 2206T, Filobasidium floriforme CBS 6240 and Sporidiobolus salmonicolor CBS 5937) were analyzed by HPLC for osmolytes released when exposed to hypo-osmotic shock. The strains differed in the pattern of response of osmolyte release. Only three strains released most of their osmolytes rapidly within 5 min, while C. macerans CBS 2206T and R. muculaginosa CBS 5951 retained most of the osmolytes intacellularly. This suggests that there might be different mechanisms of osmolyte release in basidiomycetous yeasts. A few strains of basidiomycetous yeasts (C. neoformans US I1, R. mucilaginosa CBS 5951, C. laurentii 1F, C. macerans CBS 2206T, F. floriforme CBS 6240, C. neoformans CBS 0132, C. laurentii CBS 0139, S. salmonicolor CBS 5937 and Filobasidium capsuligenum CBS 4381) were grown in soil cultures of different field capacity (100%, 25%, 10% and 5%) and evaluated for their survival in this environment. All the strains grew at 100% field capacity. Strains R. mucilaginosa CBS 5951, F. floriforme CBS 6240 and F. capsuligeum CBS 4381 also showed growth in soil at 25% field capacity. However, strains C. neoformans US I1, C. laurentii US 1F, C. macerans CBS 2206T and C. laurentii CBS 0139 did not grow at this moisture content but survived up until the end of the experimental period. At lower soil moisture content (5% and 10% field capacity), the yeast strains either showed survival or decreased viability towards the end of the experimental period. Strain C. neoformans US I1, C. laurentii US 1F, C. macerans CBS 2206T and R. mucilaginosa CBS 5951 and F. floriforme CBS 6240 showed survival at both 5% and 10% field capacity. However, strain F. neoformans CBS 0132, C. laurentii CBS 0139, F. capsuligenum CBS 4381 and S. salmonicolar CBS 5937 showed a decrease in viability after either 2 or 5 days of incubation. No relationship could be found between the type and number of intracellular osmolytes accumulated when exposed to osmotic stress (0.95aw NaCl) and the ability to grow and survive in soil with lower moisture content. Similarly, the ability of the yeasts to grow and survive in soil with lower moisture content did not correlate with their minimum aw for growth in a liquid medium. It was speculated that other factors, such as the physico-chemical composition of the soil, may also play a role in the survival of a particular yeast species in soil. This study has shown that the responses of basidiomycetous yeasts to reduced aw are physiologically similar to the ascomycetous yeasts. The types of osmolytes accumulated are similar but the basidiomycetous yeasts appear to be more sensitive to reduced aw and they tolerate NaCl better than sorbitol whereas the ascomycetous yeasts tolerate high sugar environments better. This is in agreement with the environments where these yeasts are usually found.