Yeast, the single-celled fungi, represent a surprisingly large and diverse group of microorganisms. While many people associate the term with baking or brewing, it is a broad classification within the Fungi kingdom. At least 1,500 species of yeast have been formally identified and described. This number is constantly shifting as new species are discovered and genetic sequencing reveals previously unknown relationships among existing strains.
The Vast Diversity of Yeast Species
The estimated number of known yeast species, currently standing around 1,500 to 2,000, represents only a small fraction of the total biodiversity. The true global number of yeast species is conservatively estimated to be around 150,000, meaning less than one percent have been discovered so far. As scientists explore more diverse habitats, the number of formally identified species continues to grow.
Yeasts do not form a single, unified taxonomic group; instead, the term describes a single-celled lifestyle found across two major phyla of the Fungi kingdom: Ascomycota and Basidiomycota. The majority of yeasts, including the most commonly known species, fall under the Ascomycota phylum, specifically within the Saccharomycotina subphylum. Most yeast species are non-pathogenic and live freely in nature, inhabiting environments like soil, plant surfaces, and water.
Essential Roles in Food Production
The most famous and economically significant yeast is Saccharomyces cerevisiae, often called baker’s or brewer’s yeast. This species is considered one of the earliest domesticated microorganisms, with archaeological evidence pointing to its use by humans for over 5,000 years. Its importance stems from its metabolic process, known as fermentation, where it converts carbohydrates like glucose and fructose into ethanol and carbon dioxide.
In baking, the carbon dioxide gas produced by S. cerevisiae during fermentation is trapped within the dough, causing it to rise and giving bread its light, airy texture. In brewing and winemaking, the yeast’s production of ethanol is the desired outcome, creating alcoholic beverages like beer, wine, and spirits. Beyond alcohol, S. cerevisiae generates a variety of other metabolic by-products, such as esters and higher alcohols, which contribute significantly to the unique flavor and aroma profiles of fermented foods and beverages.
Specific strains of this yeast are sold as nutritional yeast, valued for being rich in B vitamins. Fermentative yeasts improve the nutritional value and texture of other foods, such as in the fermentation of cocoa beans before chocolate production. The widespread use of S. cerevisiae across various industries demonstrates its economic significance.
Yeasts That Affect Human Health
While many yeasts are beneficial, a smaller number are opportunistic pathogens that can cause infections in humans, particularly those with compromised immune systems. The most common example is Candida albicans, which is often a normal, harmless resident of the human gut, mouth, and skin microbiota. However, a disruption in the microbial balance or a weakened immune response can allow C. albicans to overgrow and cause infections known as candidiasis, which includes common conditions like thrush and yeast infections.
C. albicans is a dimorphic fungus, meaning it can switch between a single-celled yeast form and a filamentous form called hyphae. It secretes hydrolytic enzymes that help it adhere to host tissues and invade the body. Another significant pathogenic yeast is Cryptococcus neoformans, which affects immunocompromised individuals, causing cryptococcosis, a severe fungal infection of the lungs and central nervous system. C. neoformans virulence is enhanced by its ability to synthesize a protective capsule and melanin.