Microbiology^{0}

The most recent significant impact of yeast on human beings occurred within the last 100 years, and was at the root of my initial decision to choose them as my topic domesticated organism. As a life sciences student interested in human disease, my education has focused heavily on molecular biology and the physiology of microorganisms. In such studies, a few microscopic organisms (both bacterial and fungal) have stood out as model organisms for extensive research. Such research reveals much of the biological process common to all cellular organisms (including humans), as well as illuminating a vast amount relating specifically to the particular organism studied.

Saccharomyces cerevisiae is likely the most studied eukaryotic organism alive, at least at the microscopic or molecular level. Scientists employed it as a model organism for well over a century, and used it to glean vast amount of knowledge about eukaryotic cells. An enormous amount of what we as a species understand about the biochemical and microbiological mechanisms of the fermentation pathway, respiratory pathways, enzyme regulation and metabolism, metabolic transport, diffusion (both active and facilitated), and both eukaryotic and fungal genetics derive directly from work with yeast.[1]

Rather than cover all of those in detail (a work which could and has taken up entire books), I will focus on one as an example: the respiratory pathway. Very simply put, the respiratory pathway describes the process by which eukaryotic cells utilize sugars and oxygen to produce energy and water as well as a host of nutrients, and eventually release carbon dioxide. A different outcome is available in the absence of oxygen, one that produces ethanol (alcohol) and is the basis for fermentation. Yeast played a crucial role in the discoveries of the Krebs Cycle (the main cycle in respiration),[2]^,[3] the discovery of the molecule acetate (crucial for an extremely large number of pathways within respiration),[4] cytochrome (a pigment that allowed an incalculable amount of experiments to become possible with absorption spectroscopy),[5] and oxidation (a chemical process fundamental to all life on earth),[6] just to name a few. Further reading into each of those biochemical and microbiological processes can be found in any Biochemistry textbook, such as Reginald Garrett and Charles Grisham’s Biochemistry (2012).

A very nice table of discoveries involving just the Krebs Cycle (also known as the TCA Cycle) is shown below, giving a brief chronology from 1931 to 1960.[7]

The discovery of yeast and its role in fermentation was performed by the French scientist Louis Pasteur. It is no exaggeration to say that Pasteur belongs in the discussion as the single biggest lifesaver in the history of mankind (along with Fritz Haber and Carl Bosch – German chemists whose work synthetic fertilizers have helped to feed billions, though ironically they each helped kill millions through their work for the Germans in WWI, and Norman Borlaug – who helped create high yield wheat and has been credited with over a billion lives saved) as his work proved the validity of vaccines (first experimented with by Edward Jenner), established the field of microbiology, and was one of the creators of germ theory. Pasteur disproved the theory of spontaneous generation and showed that biogenesis was the key to explaining how life originates. Before Pasteur’s work, many people believed that spontaneous generation of organisms was the reason for food going bad or spoiling. Pasteur set up simple experiment to disprove that theory: he boiled two flasks of nutrients and then sealed one of them from the air. After a certain period of time the flask that was open to the air contained bacterial growth while the sealed flask remained sterile. This experiment helped solidify germ theory and opened the door for a whole new world of scientific and medical research against human diseases.

Pasteur also discovered that by aerating a sample of yeast in a culture, growth of the microorganism went up but the fermentation (and subsequent ethanol production) went down. Named the Pasteur Effect in his honor, it works because yeast are facultative anaerobes. When yeast have access to oxygen they can readily use it and perform normal metabolic processes. However, when oxygen is not present in their environment they switch biochemical gears and perform fermentation.