Yeast

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.