A microbe, short for microorganism, is any living thing too small to see without a microscope. That covers an enormous range of life: bacteria, archaea, fungi, protozoans, algae, and, by most definitions, viruses. Your body contains roughly as many bacterial cells as human cells, a near 1:1 ratio, which gives some sense of how deeply woven microbes are into everyday life. Most are harmless or actively helpful. Only a small fraction cause disease.
The Main Types of Microbes
Microbes fall into several broad groups, each with a fundamentally different biology.
- Bacteria are single-celled organisms without a nucleus. They come in a variety of shapes (rods, spheres, spirals) and live in virtually every environment on Earth. A common bacterium like E. coli measures roughly 1 by 2 micrometers, about 500 times smaller than the period at the end of this sentence.
- Archaea look superficially similar to bacteria under a microscope, but their internal chemistry is distinct. Their cell walls lack the rigid compound found in bacterial walls, and their cell membranes are built from a different type of fat molecule. These differences are so fundamental that archaea are classified as an entirely separate domain of life. Many archaea thrive in extreme environments, though they also live in soil, oceans, and the human gut.
- Fungi include yeasts and molds at the microscopic scale, plus mushrooms at the visible scale. Unlike bacteria, fungal cells have a nucleus and internal compartments, making them more structurally complex. Some fungal cells can grow remarkably large: the human pathogen Cryptococcus neoformans, when lodged in the lungs, can balloon up to 100 micrometers in diameter, big enough to overwhelm the immune cells trying to engulf it.
- Protozoans are single-celled organisms with a nucleus that often behave like tiny animals, moving through water and consuming other microbes. Amoebas and the parasites that cause malaria belong to this group.
- Algae are photosynthetic microbes found in water and on damp surfaces. They produce a significant share of the oxygen in Earth’s atmosphere.
- Viruses are the outliers. They are far smaller than bacteria and cannot reproduce on their own. A virus must hijack a living cell to copy itself. Whether viruses count as “alive” is a long-running scientific debate with no clean resolution. They combine features of both living and non-living matter: they evolve and reproduce (animate traits), but outside a host cell they exist in a completely inert state, more like a particle than an organism. Most microbiologists include viruses in the study of microbiology regardless of where they land on the alive-or-not question.
How Small Are They, Really?
Size varies wildly across the microbial world. The smallest known bacteria, such as Mycoplasma species, are about 0.2 micrometers across. That’s roughly 50 times smaller than a red blood cell. At the other extreme, the bacterium Thiomargarita namibiensis reaches 750 micrometers in diameter, large enough to see with the naked eye. Between those two endpoints lies a 10-billion-fold difference in volume.
Viruses are smaller still, typically measured in nanometers (thousandths of a micrometer). Most fall in the range of 20 to 300 nanometers. At that scale, gravity is irrelevant to their movement. They drift through fluids carried entirely by the motion of surrounding molecules.
What Microbes Do in Your Body
The trillions of microbes living on and inside you, collectively called the microbiome, perform work your own cells cannot. Humans are incapable of synthesizing most vitamins, but gut bacteria pick up the slack. They produce vitamin K along with nearly all the water-soluble B vitamins: B1 (thiamine), B2 (riboflavin), B3 (niacin), B5 (pantothenic acid), B6, B7 (biotin), B9 (folate), and B12. These microbially produced vitamins don’t just feed neighboring gut cells. They also contribute, in a smaller but measurable way, to your overall vitamin status.
Gut microbes also generate short-chain fatty acids by breaking down dietary fiber your own digestive enzymes can’t touch. These molecules help maintain the gut lining, regulate inflammation, and influence immune function. The composition of your microbiome shifts across your lifetime. In infancy, the dominant bacteria tend to produce more folate and vitamin K2, reflecting the nutritional demands of early development. In adulthood, the microbial community reshuffles around different metabolic priorities, with different species trading vitamins among themselves in cooperative networks.
How Microbes Drive Earth’s Ecosystems
Without microbes, the planet’s nutrient cycles would grind to a halt. Two of the most critical are the carbon cycle and the nitrogen cycle, and microbes are central to both.
In the carbon cycle, fungi and bacteria decompose dead plants and animals, converting complex organic matter back into simpler compounds. White-rot fungi are especially important because they are among the only organisms that can break down lignin, the tough structural compound in wood. The breakdown products combine with proteins and sugars to form humus, a stable form of carbon that stays locked in soil for long periods. On the flip side, certain archaea called methanogens produce methane under oxygen-free conditions, while methane-eating bacteria in oxygen-rich zones convert that methane into carbon dioxide and water, significantly reducing how much of this potent greenhouse gas reaches the atmosphere.
In the nitrogen cycle, specialized bacteria and archaea pull nitrogen gas out of the air and convert it into ammonia, a form that plants can absorb. This process, called nitrogen fixation, is the primary way new usable nitrogen enters ecosystems. Other microbes then convert ammonia into nitrate through a two-step chain, while still others run the cycle in reverse, turning nitrate back into nitrogen gas and releasing it to the atmosphere. Every step in this cycle depends on microbes. Without them, soils would lose their fertility and food production would collapse.
How Harmful Microbes Cause Infection
The small minority of microbes that cause disease use a surprisingly diverse toolkit. The first step is usually attachment: many bacteria use hair-like structures called pili to latch onto the lining of your throat, gut, or urinary tract. Once attached, they need to outcompete your immune system. Some bacteria surround themselves with a slippery capsule that makes it harder for immune cells to grab and engulf them. Others go a step further and survive inside the very immune cells sent to destroy them, either by breaking out of the internal compartment where they’ve been trapped or by preventing that compartment from activating its killing machinery.
Nutrients are another battleground. Your body deliberately keeps iron locked away inside transport proteins, starving invading bacteria of a mineral they need to grow. Pathogenic bacteria counter this by releasing specialized molecules that pry iron loose from those proteins. Some bacteria also produce potent toxins that damage tissues directly, and others evade long-term immunity by constantly mutating the molecules on their surface so your immune system no longer recognizes them.
Life in Extreme Environments
Microbes live in places that would instantly kill any plant or animal. The current known temperature range for microbial life stretches from -25°C (in Antarctic conditions) to 130°C (in deep-sea hydrothermal vents). Active metabolism has been documented at 122°C in a methane-producing archaeon and at -20°C in bacteria isolated from Siberian permafrost.
The pH range is equally staggering. Two species of archaea isolated from a volcanic hot spring in Japan survive at pH values just below zero, more acidic than battery acid. At the opposite extreme, a bacterium found in a mineral spring in California grows at pH 12.5, more alkaline than household bleach. These extremophiles aren’t just curiosities. They reveal how flexible the basic chemistry of life can be and help scientists understand where life might exist beyond Earth.
Microbes in Food and Industry
Humans have relied on microbes for thousands of years, often without knowing it. Lactic acid bacteria drive the fermentation of yogurt, cheese, sauerkraut, kimchi, and soy sauce. Yeasts produce the alcohol in beer and wine and the carbon dioxide that makes bread rise. Kefir and kombucha depend on mixed communities of bacteria and yeast working together. Fungal fermentation is behind tempeh and many traditional Asian soy products.
Beyond food, bacteria are used industrially to produce acetic acid (the key component of vinegar) and a range of organic compounds. In environmental cleanup, bacteria break down pollutants including polychlorinated biphenyls and polycyclic aromatic hydrocarbons, toxic compounds found at contaminated industrial sites. This process, called bioremediation, uses the natural metabolic abilities of microbes to detoxify soil and water at a fraction of the cost of chemical treatment.