Yeasts are single-celled fungi, microscopic organisms that belong to the same biological kingdom as mushrooms and molds but grow as individual round or oval cells rather than as branching filaments. They live virtually everywhere on Earth, from ocean depths to mountain air to the surface of your skin, and humans have relied on them for thousands of years to make bread, beer, and wine. Despite their tiny size, yeasts play outsized roles in ecosystems, in industry, and inside your own body.
How Yeasts Differ From Other Fungi
Fungi grow in two major ways: as long, thread-like filaments (the familiar fuzzy mold on old bread) or as discrete single cells. Yeasts take the single-cell route. Each yeast cell lives independently, dividing on its own rather than forming the branching networks that molds and mushrooms build. This makes yeasts unusual in the fungal world, though the distinction isn’t always clean cut. Some fungal species can switch between a filamentous form and a yeast form depending on their environment.
Like all fungi, yeasts are eukaryotes, meaning their cells contain a nucleus and internal compartments, just like human cells. Fungi diverged from animals roughly one billion years ago, making them more closely related to us than to bacteria or plants. A typical yeast cell measures about 3 to 10 micrometers across, roughly ten times larger than most bacteria but still far too small to see without a microscope.
How Yeasts Reproduce
Most yeasts reproduce asexually through a process called budding. A small bump forms on the surface of the parent cell, gradually grows, and then pinches off as a new daughter cell. Under ideal lab conditions, a single yeast cell can complete this cycle in about 90 minutes, which is one reason yeasts multiply so quickly in warm dough or sugary liquids. A smaller number of yeast species reproduce by fission instead, splitting cleanly down the middle like bacteria do.
Yeasts also have a sexual cycle. Two yeast cells of compatible mating types can fuse together, combining their genetic material into a single cell with a double set of chromosomes. When nutrients run low, particularly nitrogen, that fused cell can undergo a special division that shuffles its genes and produces four spores. Each spore can then germinate into a new yeast cell. This sexual shuffling generates genetic diversity, helping yeast populations adapt to changing environments.
Fermentation: Turning Sugar Into Alcohol and Gas
The trait that made yeasts indispensable to human civilization is fermentation. When oxygen is scarce, yeast cells break down sugars like glucose and convert them into ethanol (alcohol) and carbon dioxide. This is the same basic chemistry behind rising bread dough, bubbling beer, and aging wine.
In bread baking, the carbon dioxide gas gets trapped in gluten, creating the airy texture of a loaf. The small amount of alcohol produced evaporates during baking. In brewing and winemaking, the ethanol is the desired product, and the carbon dioxide either escapes or is retained for carbonation. Yeasts can also use oxygen when it’s available, burning sugars more efficiently for energy, but it’s their ability to thrive without oxygen that humans learned to exploit millennia ago.
Thousands of Years of Human Use
The yeast species most familiar to humans is commonly called baker’s yeast or brewer’s yeast. Archaeological evidence suggests people were using yeast to make bread as early as 10,000 BCE, though the oldest confirmed leavened breads date to the second millennium BCE in Egypt. Wine has a similarly ancient history: chemical traces of wine have been identified in pottery jars from Iran dating to around 5,400 BCE, and yeast DNA has been recovered from an Egyptian wine jar dated to 3,150 BCE.
For most of that history, nobody understood what yeast actually was. It wasn’t until 1860 that Louis Pasteur demonstrated that living microorganisms were responsible for fermentation. By the 1890s, winemakers began using controlled yeast starter cultures, and bakers started sourcing their yeast from brewery leftovers. Today, baker’s yeast is the most common yeast species found in both commercial bread production and traditional sourdoughs.
Beyond food, this same species has become one of the most important organisms in biotechnology and basic science. It was the first eukaryote to have its entire genome sequenced, and researchers routinely use it to study fundamental cell processes that are shared across all complex life, including humans. It also plays a growing role in biofuel production, where engineered yeast strains convert plant sugars into ethanol at industrial scale.
Where Yeasts Live in Nature
Yeasts are primarily free-living decomposers. In the wild, they break down organic matter, generate biomass that feeds other organisms, and help cycle nitrogen and sulfur through ecosystems. Researchers have cataloged over 340 species of yeasts and yeast-like fungi in natural environments, but the true number is likely far higher since many habitats remain undersampled.
Their range is extraordinary. Yeasts have been found in ocean depths, on the sea surface, in rivers and glaciers, in desert soils and polar ice, and even in the stratosphere, where they endure extreme cold, intense ultraviolet radiation, and near-vacuum pressures. They colonize the surfaces of fruits, the bark of trees, the guts of insects, and the skin of animals. Essentially, if an environment has even a trace of organic carbon and moisture, yeasts are probably there.
Yeasts in the Human Body
Your body hosts a resident community of fungi, sometimes called the mycobiome, and yeasts make up a significant part of it. Healthy humans harbor at least 66 fungal genera and roughly 180 species across the gut, lungs, and skin. In the gut specifically, the most abundant yeasts tend to be baker’s yeast and various Candida species, along with genera like Malassezia and Cladosporium.
Under normal circumstances, these commensal yeasts coexist peacefully with bacteria and with your immune system. Gut fungi help train the immune system and contribute to maintaining a stable internal environment. Candida albicans, for example, is routinely detected in the feces of healthy people and is considered a normal part of the gut ecosystem. In immunocompetent individuals, it maintains a balanced relationship with the body’s other microorganisms and causes no harm.
When Yeasts Cause Infections
Problems arise when the balance tips. Candida species are the most common cause of fungal infections in humans. Vaginal yeast infections, oral thrush, and skin infections in warm, moist folds of the body are all typically caused by Candida overgrowth, often triggered by antibiotic use, a weakened immune system, or changes in the body’s normal microbial balance. In people with severely compromised immunity, Candida can enter the bloodstream and become life-threatening.
Another yeast genus, Cryptococcus, causes serious lung and brain infections, primarily in immunocompromised individuals. And Candida auris, a species first identified in 2009, has rapidly emerged as a global hospital-acquired infection that resists multiple antifungal drugs, making it a significant public health concern. Malassezia, a yeast that naturally lives on human skin, can overgrow and contribute to dandruff and certain types of dermatitis.
Nutritional Yeast as a Food Supplement
Nutritional yeast is a deactivated form of yeast, meaning the cells have been killed and can no longer ferment. It’s sold as yellow flakes or powder with a savory, slightly cheesy flavor that makes it popular as a seasoning, especially among people following plant-based diets. Two tablespoons provide about 5 grams of protein and 2 grams of fiber.
The nutritional profile depends heavily on whether the product is fortified. Unfortified nutritional yeast is naturally rich in riboflavin (one serving delivers 180% of the daily value) but contains only modest amounts of other nutrients. Fortified versions, which are more common on store shelves, can contain high levels of several B vitamins including B12, B6, niacin, thiamin, and folate. This makes fortified nutritional yeast particularly useful for vegans, since B12 is otherwise found almost exclusively in animal products.