Saccharomyces cerevisiae, the budding or baker’s yeast, is a single-cellular eukaryotic microorganism. It requires various physical and nutritional conditions to grow and thrive in various environments. With proper physical conditions and nutrients, yeast cells can double every 100 minutes (“Saccharomyces cerevisiae,” n.d.). Its average replicative lifespan is around 36 cell divisions (“Saccharomyces cerevisiae,” n.d.).
Two critical physical requirements are:
- Saccharomyces cerevisiae requires acidic pH to grow. The optimum pH for S. cerevisiae is 4−6 (Olivero, 1982, p. 2726). Its growth rate drops dramatically when pH is increased above 6.5 (Olivero, 1982, p. 2726). This requirement allows the budding yeast to thrive in soil, which is mostly acidic all over the world. Inside the human body, the colon is slightly acidic, which allows the yeast to grow. If the pH balance in human body becomes altered and the pH levels become more acidic, there is a chance of yeast overgrowth. On the other hand, the pH requirement of S. cerevisiae limits the ability of the budding yeast to grow at alkaline pH and prevents it from spreading to aqueous environments that are mostly neutral or basic. This is the reason why chlorine is usually added to swimming pools to make the pH of the water slightly alkaline, preventing yeast from growing in the water.
- S. cerevisiae can grow both aerobically and anaerobically. It uses respiratory and fermentation metabolism to generate energy under aerobic and anaerobic conditions, respectively (Snoek, 2007, p. 1). This allows the budding yeast to grow in both O2-abundant and O2-deficient environments. The yeast, when buried in soil or located in the inside of human organs where the O2 levels are low, can still grow and help maintain a balanced microorganism environment. This anaerobic growth, however, requires the presence of certain sterols and unstatuarated fatty acids (Snoek, 2007, p. 1) and thus limits the ability of S. cerevisiae to thrive.
Two critical nutritional requirements are:
- S. cerevisiae, depending on the strain and aerobic or anaerobic condition, can grow on various carbon sources, including glucose, maltose, trehalose, fructose, galactose, and sucrose (“Saccharomyces cerevisiae,” n.d.). Glucose is the simplest form of sugar and is abundant in nature. Maltose is a common product from the hydrolysis of starch. Trehalose is the blood-sugar of shrimps and many insects. Galactose can be generated from the breakdown of lactose. Sucrose and fructose are abundant in canes and fruits, respectively, allowing the yeast to thrive on the skins of canes and fruits. However, because the majority of the yeast sits in the gastrointestinal tract inside of human body, the inability of the yeast to grow on lactose limits its growth in human body.
- S. cerevisiae can use ammonia, urea, most amino acids, small peptides, and nitrogen bases as a nitrogen source. It, however, cannot use nitrate as a nitrogen source, because it is unable to reduce nitrate to ammonium ions (“Saccharomyces cerevisiae,” n.d.). Ammonia and urea, generated from animal waste and terrestrial organisms, are very abundant in nature, allowing the yeast to thrive in various environments. To grow S. cerevisiae in a laboratory, the growth media is often supplemented with ammonium ions and nitrogen bases as the nitrogen sources. The inability of S. cerevisiae to use nitrate, however, may limit its growth ability and prevent it from spreading into an environment lack of nitrate reductase.
Reference
Olivero, I., Ruiz-Macias, C., Chordi, A., and Peinado, J. M. (1982). Effect of External pH on the Growth of Saccharomyces cerevisiae fermenting maltose in batch and continuous culture. Biotechnology and Bioengineering, 24, 2725-2729.
Snoek, I. S. Ishtar, and Steensma, H. Yde. (2007). Yeast, 24, 1-10.
Saccharomyces cerevisiae. (n.d.). Retrieved April 29, 2013 from the Wiki: http://en.wikipedia.org/wiki/Saccharomyces_cerevisiae
