Yeast

Yeasts are single-celled fungi , belonging mainly to the Ascomycetes, that serve as nutrient recyclers in nature, but are also important in industry, biotechnology , and as the agents of disease in humans. The term yeast is generically used in reference to many species of single-celled, budding fungi, including Saccharomyces—used in baking and brewing —and Candida—an infectious yeast common in people with compromised immune systems, such as AIDS victims.

Life cycle

Yeasts secrete enzymes that break down carbohydrates (through fermentation ) to yield carbon dioxide and alcohol . The source of carbohydrates are either living hosts or non-living hosts such as rotting vegetation, or the moist body cavities of animals. Yeasts are considered by some scientists to be closely related to the algae , lacking only in photosynthetic capability—perhaps as a result of an evolutionary trend toward a lifestyle dependent upon host nutrition . Ecologically yeasts are decomposers that secrete enzymes which dismantle the complex carbon compounds of plant cell walls and animal tissues, which they convert to sugars for their own growth and sustenance. Yeast reproduction may involve sexual spore production or asexual budding, dependent upon surrounding conditions. Though yeasts are highly tolerant of environmental variations in temperature and acidity, they thrive in warm and moist places high in oxygen and low in carbon dioxide. Whether or not they reproduce through asexual budding depends on the favorability of surrounding conditions: when times are good, yeast clones are produced by budding. In times of environmental stress, yeasts produce spores which are capable of withstanding periods of environmental hardship—perhaps even to lie dormant, until conditions improve and the mingling of genes can take place with the spore of another yeast. This rare version of yeast reproduction provides for genetic
variation when conditions demand it, though budding is the predominant mode of yeast reproduction.

The importance of yeast for humans

People have been using yeast in bread baking for centuries, but suffering from its scourges for much longer. The biochemical by-products of yeast sugar metabolism—carbon dioxide and alcohol—are essential in baking and brewing. Bakers yeast (Saccharomyces cerevisiae), when added to baker's dough, produces carbon dioxide pockets that make bread rise. Brewer's yeast, another strain of Saccharomyces, takes advantage of the yeasts characteristic of switching to anaerobic fermentation when deprived of oxygen to produce alcohol as a by-product of incomplete sugar breakdown. Yeasts that occur naturally on the skins of grapes also play a vital role in fermentation—converting the sugars of grapes into alcohol for wine production.

Yeasts also comprise some of the natural microbial flora residing in and upon animals, including humans. The yeast species Candida albicans is perhaps the most notorious of the yeast inhabitants of the human body, responsible for the affliction Candidiasis, which may take many forms. Through normal health and hygiene, Candida is held in check by the populous and benign bacterial residents of our skin and mucous membranes. But in instances of compromised health, Candida albicans can result in skin sores (such as Thrush), urogenital tract infections (such as vaginitis), and internally, endocarditis (heart muscle infection ), inflammation of the spleen, liver, kidneys and lungs. Victims of AIDS are particularly susceptible to Candidiasis.

Biotechnology and yeast

Yeasts are rising stars in the toolbox of biotechnologists. Early in the development of biotechnology, which used cells as recipients of transplanted genes, bacteria were the organism of choice. However, limitations involving differences between bacterial cells and our own have relegated bacteria to a second place behind yeasts. Both yeast cells and human cells are eukaryotic—possessing a nucleus and membrane-bound organelles such as ribosomes and mitochondria. As biotechnology techniques have progressed over the last decade, yeasts have come to the fore as host cells for human gene implantation and as potential surrogate cells for housing human chromosomes. Yeasts have also been subjected to the alteration of their own genes, as biotechnologists attempt to develop strains of yeast more efficient in the metabolism of sugars for food and industrial applications.

By studying yeast genetics , scientists hope to gain insight into how the genes of all eukaryotic cells, including our own, function. In 1993 the first yeast chromosome was completely mapped, and the function of each of the 182 genes has yielded insight into such vital genetic processes as mutation repair, enzyme production, and cellular division regulation. These findings may yield insights into such fundamental human health issues as cancer and the aging process. Yeasts have also been transformed through genetic engineering to produce the first genetically engineered vaccine for hepatitis B. Insulin and hemoglobin are also being commercially manufactured by yeast cells which have been re-coded by inserted human genes. Yeasts are a hopeful host cell for housing the entire human genome in an international effort to decipher the gene library of human cells.

Resources

books

Davis, Joel. Mapping the Code: The Human Genome Project and the Choices of Modern Science. Wiley, 1990.

Gross, Cynthia S. The New Biotechnology: Putting Microbes to Work. New York: Lerner, 1988.

Hanlin, Richard, and Miguel Ulloa. Atlas of Introductory Mycology. 2nd Ed. H. W. Wilson Co., 1988.

Krasner, R.I. The Microbial Challenge: Human-Microbe Interactions. Washington: American Society for Microbiology, 2002.

Sagan, Dorion. Garden of Microbial Delights: A Practical Guide to the Subvisible World. Harcourt Brace Jovanovich, 1988.

Stephens, R. B., ed. Mycology Guidebook. Mycology Guidebook Committee, Mycological Society of America, University of Washington Press, 1981.

periodicals

Chan, K.C., A. Csikasz-Nagy, et al. "Kinetic Analysis of a Molecular Model of the Budding Yeast Cell Cycle". Molecular Biology of the Cell 11 (2000): 369-391.

Hoffman, M. "Yeast Biology Enters a Surprising New Phase." Science (March 20, 1992): 1210-1511.

Jeffrey Weld

KEY TERMS

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Candidiasis

—Infection of the yeast Candida albicans which may take many forms depending upon the site of infection from inflammation of the heart (endocarditis) to throat and lung infections.

Eukaryote

—A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.

Human genome

—The entire human gene library, consisting of an estimated 100,000 genes encoded in the tightly woven DNA of 23 chromosomes.

Microbial flora

—Benign, naturally occurring of bacteria and fungi found on, and in, the human body, including the intestinal tract, the mucous membranes, and the skin.