Browsing by Author "Bester, Reinhard"

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    Growth and survival of Saccharomyces cerevisiae in soil
    (Stellenbosch : University of Stellenbosch, 2011-10) Bester, Reinhard; Botha, Alfred; Wolfaardt, Gideon M.; University of Stellenbosch. Faculty of Science. Dept. of Microbiology.
    ENGLISH ABSTRACT: Saccharomyces cerevisiae is commonly associated with the wine industry. However, this yeast was also isolated from soils not associated with vines. Despite the fact that S. cerevisiae is not perceived as an autochthonous soil yeast, its interaction with other soil microbiota suggests the contrary. Aside from a few in vitro studies, the fate of S. cerevisiae in soil is largely unknown. This may partly be ascribed to the lack of reliable methods to enumerate fermentative yeasts in soil. Consequently, we evaluated an enumeration procedure for fermentative yeasts in soil, whereby yeast malt extract (YM) agar plates containing selective agents, were incubated in anaerobic jars before the colonies were enumerated. This procedure proved to be selective for fermentative yeasts, such as industrial strains of S. cerevisiae. We then commenced studying the growth and survival of S. cerevisiae in soil differing in moisture content and nutrient levels, using S. cerevisiae strain S92 and the genetically modified strain S. cerevisiae ML01, as well as two autochthonous soil yeasts, Cryptococcus laurentii and Cryptococcus podzolicus. The yeast strains were each inoculated into three series of microcosms containing sterile soil with a moisture content of ca. 30% (v/w), a moisture content of ca. 15% (v/w), or a moisture content of ca. 30% supplemented with nutrients used in agriculture. Growth of each strain was monitored for a period of 48 days and all the yeasts were found to grow or survive under these conditions, up until the end of the incubation period. Generally, the cryptococci reached larger population sizes in the soil than the Saccharomyces strains, which may be due to their ability to utilize a wider range of carbon sources and to survive in semi-arid soils. Aside from cell numbers observed in nutrient supplemented soil, in which S. cerevisiae ML01 reached higher numbers than S92, there was no significant difference between the growth and survival of the Saccharomyces strains. In all the microcosms, metabolic rates, as determined by measuring CO2 emissions from soil, reached a maximum within the first day and then declined over the remainder of the trial, possibly due to depletion of nutrients. Differences in CO2 emissions from the different series of microcosms were attributed to different metabolic rates and energy expenditure needed to maintain yeast populations under different conditions. Each of the above-mentioned yeasts was subsequently inoculated in a microcosm prepared from non-sterile soil and monitored using selective enumeration procedures. The Saccharomyces strains were enumerated using the above-mentioned soil dilution plates incubated in anaerobic jars. The presence of natural soil biota caused a decrease in viable yeast numbers for all strains and this was ascribed to competition with and predation by other soil borne organisms. Further evidence for competition and/or amensalism impacting on Saccharomyces populations in soil was obtained when monitoring co-cultures of Saccharomyces with C. laurentii 1f and C. podzolicus 3f in soil microcosms, revealed a significant reduction in Saccharomyces numbers during a 28 day incubation period. However, when the two Saccharomyces strains were cultured in soil microcosms inoculated with a protistan predator, populations of both strains increased and remained at these high levels for the duration of the trial. These findings point to a possible symbiosis between Saccharomyces and the protista whereby the predators ensure continuous nutrient cycling within the soil microcosms. In the final part of the study, epifluorescence microscopy revealed that, similar to known soil cryptococci, the two Saccharomyces strains were able to form biofilms in oligotrophic conditions. The results of this study showed that in the presence of natural soil microbes, no differences exist between the growth and survival of S. cerevisiae S92 and S. cerevisiae ML01. Also, the findings point to a natural niche for this species somewhere in the soil habitat.