Microbes are living organisms too small to see without a microscope. They include bacteria, archaea, fungi, protists, and (by some definitions) viruses. They are the most abundant life forms on Earth, with an estimated 4 to 6 × 10³⁰ prokaryotic cells in the biosphere alone. They live in soil, oceans, the atmosphere, deep inside rock, and all over your body.
The Major Groups of Microbes
Microbes span multiple branches of the tree of life. In the 1970s, microbiologist Carl Woese proposed reorganizing all life into three domains: Bacteria, Archaea, and Eukarya. Microbes exist in all three.
Bacteria are single-celled organisms without a nucleus. They reproduce by splitting in two, a process called binary fission, and they can double their population in as little as 20 minutes under ideal conditions. A typical bacterium is about 1 to 5 micrometers long, roughly 50 times smaller than the width of a human hair. Their cell walls are built from a mesh of proteins and sugars called peptidoglycan.
Archaea look similar to bacteria under a microscope, but they’re genetically more closely related to plants and animals than to bacteria. Their cell walls are made of different sugars, they use different chemistry to build their membranes, and they include some of the most extreme survivors on the planet. The distinction between bacteria and archaea is one of the most important reclassifications in modern biology.
Fungi range from single-celled yeasts to massive underground networks of mushroom-producing organisms. Microscopic fungi, like the yeast used in bread and beer, are true microbes. Others, like molds, exist as tangles of tiny threads invisible to the naked eye until they form a visible colony.
Protists are eukaryotic microorganisms that don’t fit into the animal, plant, or fungus categories. They include amoebas, algae, and the parasites behind diseases like malaria. Despite being overlooked, protists are foundational to life on Earth: protist communities may produce around 50% of the oxygen in our atmosphere through photosynthesis, and they form the base of marine food chains.
Viruses occupy an unusual gray zone. They contain genetic material (DNA or RNA) wrapped in a protein coat, but they cannot generate energy or reproduce on their own. Outside a host cell, viruses are inert. Because of this, many biologists don’t consider them truly “alive.” Still, they’re routinely studied in microbiology and are often included in broad discussions of microbes.
Where Microbes Live
Microbes occupy every environment on Earth, including places that would kill most other organisms. Extremophile microbes have been found 6.7 kilometers inside the Earth’s crust and more than 10 kilometers deep in the ocean, under crushing pressures. One archaea species grows at 122 °C, the highest recorded temperature for any living organism. Another thrives at a pH of 0.06, an acidity far beyond battery acid. Microbes have also been found in frozen sea water at −20 °C and in highly alkaline lakes with a pH of 12.8.
More familiar habitats matter just as much. Soil teems with bacteria, fungi, and protists that form complex communities around plant roots. Ocean surface waters are rich in photosynthetic microbes that pull carbon dioxide from the atmosphere. And every surface you touch, from kitchen counters to doorknobs, hosts microbial communities.
Microbes in the Human Body
Your body carries roughly 38 trillion bacterial cells alongside about 30 trillion human cells, giving a ratio of approximately 1.3 to 1. That’s a far cry from the old claim that microbes outnumber your cells 10 to 1, a figure that persisted in textbooks for decades before a 2016 revised estimate corrected it. The total mass of all your bacteria comes to about 0.2 kilograms, roughly half a pound.
Most of these bacteria live in your colon, but microbial communities also populate your skin, mouth, airways, and reproductive tract. These communities help digest food, train your immune system, produce certain vitamins, and compete with disease-causing organisms for space and resources. The composition of your microbiome shifts with diet, medication, age, and environment.
How Microbes Drive Earth’s Chemistry
Microbes run two of the planet’s most critical chemical cycles: nitrogen and carbon.
Plants need nitrogen to grow, but they can’t use the nitrogen gas that makes up 78% of the atmosphere. Certain bacteria and archaea, called diazotrophs, convert atmospheric nitrogen into ammonia, a form plants can absorb. This process, biological nitrogen fixation, is the primary way new nitrogen enters ecosystems. Some of these microbes partner with plant roots directly. Others, like photosynthetic cyanobacteria, fix nitrogen independently, protecting their delicate enzyme machinery by doing the work only at night or inside specialized cells that block oxygen.
Carbon cycling is equally dependent on microbes. Bacteria and fungi break down dead plant and animal material, releasing carbon dioxide back into the atmosphere through decomposition. In oxygen-free environments like wetlands and landfills, a group of archaea called methanogens converts simple organic compounds into methane. On the flip side, methane-oxidizing bacteria in oxygen-rich zones convert that methane back into carbon dioxide and water, significantly reducing methane emissions. Some fungi break down tough plant compounds like lignin into stable forms of carbon that persist in soil for centuries as humus, effectively locking carbon away underground.
Microbes in Food and Industry
Humans have used microbes for thousands of years, often without knowing it. Bread, beer, wine, yogurt, cheese, soy sauce, and kimchi all depend on microbial fermentation. The yeast species used in baking and brewing converts sugars into carbon dioxide and alcohol. Lactic acid bacteria give yogurt its tang and act as natural preservatives in fermented vegetables.
Industrial applications go far beyond food. Certain soil bacteria naturally produce antimicrobial compounds, and the first antibiotics were derived from these organisms. Microbes are also used to produce enzymes for laundry detergent, to break down environmental pollutants like pesticides and industrial chemicals, and to generate biofuels. Bacteria employ specialized enzymes to dismantle complex toxic molecules into simpler, harmless compounds, a capability increasingly harnessed for environmental cleanup.
Microbes That Cause Disease
Only a small fraction of microbial species cause illness in humans. Pathogenic bacteria are responsible for infections like tuberculosis, strep throat, and foodborne illness. Pathogenic protists cause malaria, amoebic dysentery, and sleeping sickness. Certain fungi cause infections ranging from athlete’s foot to serious lung infections in people with weakened immune systems. Viruses cause influenza, COVID-19, measles, and many other diseases.
The vast majority of microbes, however, are harmless or beneficial. Even within a single species, only certain strains may be dangerous. The balance between helpful and harmful microbes, both in the environment and on your body, is one of the central themes of modern microbiology.