Saccharomyces pastorianus
Classification
Domain: Phylum: Ascomycota Class: Saccharomycetes Order: Saccharomycetales Family: Saccharomycetaceae [Others may be used. Use NCBI link to find]
Species
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NCBI: Taxonomy |
Saccharomyces pastorianus
Description and Significance
Describe the appearance, habitat, etc. of the organism, and why you think it is important.
Genome Structure
Describe the size and content of the genome. How many chromosomes? Circular or linear? Other interesting features? What is known about its sequence?
Cell Structure, Metabolism and Life Cycle
Saccharomyces pastorianus is an elongated, elliptical cell. It has the potential for mycelial-like structure formation under proper conditions. The life cycle of this strain can be categorized as haplodiplobiontic. Budding produces multiple generations of the yeast prior to the reinitiation of copulation. The strain will undergo both budding – as a means of reproduction – and sporulation. In sporulation, an asci is bore without any fruiting body. Each asci is separately formed, with zygotes forming into ascus and diploids into ascophores.
Flocculation, the accumulation of the yeast as it precipitates out of solution, has a unique mechanism in Saccharomyces pastorianus as it is a bottom-fermenting yeast. During the stationary growth phase, lectins emerge from the cell surface and bind to one another. This occurs only once the concentration of sugar in the surrounding aqueous solution is low enough, as high sugar concentrations inhibit flocculation. The pH must also be in a range just above the isoelectric point and cells must be in the presence of calcium. Another unique feature of the cell is that it is believed to be an allopolyploid. It contains multiple sets of chromosomes from different species.
The appropriately named Saccharomyces pastorianus – saccharomyces from the latin for “sugar fungus” – utilizes sugars during metabolism. The catabolism of sugar is crucial to the survival of the yeast, though it has an affinity for glucose or maltose – which is broken down into glucose monomers by enzymes during the malting process of fermentation. In aerobic conditions respiration occurs and the cell creates energy via the citric acid cycle. This pathway becomes the mechanism for ATP production. In the absence of oxygen, anaerobic fermentation takes place. The pentose phosphate pathway is utilized at that time. The carbon taken in by the cell is utilized not only by glycolysis, but also in the synthesis of proteins and lipids.
During aerobic respiration, the cell produces carbon dioxide as a by-product of the citric acid cycle. When anaerobic fermentation occurs, a greater variety of molecules are made. Glycerol, succinate, organic acids, and ethanol are also produced. The production of ethanol is crucial to the role of Saccharomyces pastorianus as a lager yeast strain. This provided the alcohol found in fermented beverages like beer, making this yeast of value to humans.
Ecology and Pathogenesis
Habitat; symbiosis; biogeochemical significance; contributions to environment.
If relevant, how does this organism cause disease? Human, animal, plant hosts? Virulence factors, as well as patient symptoms.
References
[Deak, T. Handbook of Food Spoilage Yeasts. CRC Press. 2007. 325 pgs.]
[Dengis, P.B., Nelissen, L.R., and Rouxhet, P.G. “Mechanisms of Yeast Flocculation: Comparison of Top- and Bottom-Fermenting Strains.” Applied and Environmental Microbiology. 1995. Volume 61(2). p. 718-728.]
[Dunn, B. and Sherlock, G. “Reconstruction of the genome origins and evolution of the hybrid lager yeast Saccharomyces pastorianus.” Genome Reseach. 2008. Volume 18(10). p. 1610-1623.]
[Guilliermond, A., and Tanner, F. W. The Yeasts. John Wiley and Sons: New York. 1920. 424 pgs.]
[Olesena, K., Feldinga, T., Gjermansena, C., and Hansena, J. “The dynamics of Saccharomyces carlsbergensis brewing yeast transcriptome during a production-scale lager beer fermentation.” FEMS Yeast Research. 2006. Volume 2(4). p. 563-573.]
[Sumbali, G., and Johri, B. M. The Fungi. Alpha Science Int’l Ltd. 2005. 298 pgs.]
Author
Page authored by Mary Ellen Hoinski and Nicole Goldman, students of Prof. Jay Lennon at Michigan State University.
