Micro-organisms (also known as microbes) are life-forms that cannot be seen by the naked eye. They include archaea, bacteria, protists and some kinds of algae and fungi. Micro-organisms may exist as single cells or as a colony of cells. They are found everywhere: in the soil, in the air, in water, inside the bodies of animals (including humans)—even inside rocks. They live in almost every habitat from the poles to the equator, deserts and the deep oceans. Some are adapted to living in extreme conditions, including very hot or very cold temperatures. Micro-organisms were the earliest life forms on Earth, with fossil evidence of their existence found in rocks dating back 3.45 billion years. The study of micro-organisms is called microbiology.

Diversity

Micro-organisms are extremely diverse: some scientists say there may be a trillion (one million million, or 1 followed by 12 zeros) species. Micro-organisms also make up by far the largest number of living things on Earth. The main types are: bacteria, fungi, algae, protists and archaea. (Viruses can only survive and replicate themselves inside the cells of other living things. For this reason, they are not regarded as true micro-organisms, although they are described below.)

Discovering micro-organisms

The existence of life-forms too tiny to be visible to the naked eye was suspected from ancient times, but the scientific study of microbial life began with the use of microscope, invented in the early 17th century. Robert Hooke (1635–1703) used a microscope of his own design to study tiny insects and minute fungi—the first known micro-organisms. He published his drawings and descriptions of the observations he had made in his 1665 book, Micrographia. Antonie Van Leeuwenhoek (1632–1723), widely considered to be the father of microbiology, was the first person to observe bacteria in the 1670s, which he did using a simple single-lensed microscope of his own construction.
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In the 1850s, Louis Pasteur (1822–1895) found that micro-organisms caused food spoilage. In the 1880s, Robert Koch (1843–1910) discovered that micro-organisms caused the diseases tuberculosis, cholera and anthrax. It was not until the end of the 19th century and the early years of the 20th—by which time viruses, along with the action of bacteria in the soil, had been identified and studied—that the diversity of the microscopic world first became known to science.

Micro-organisms are essential tools in a variety of sciences. They can be grown rapidly in large numbers and are easily manipulated. Micro-organisms can then be used for creating medicines and treating diseases.

Some micro-organisms—a number of kinds of bacteria, protists and fungi—are pathogens, the cause of many infectious diseases in humans and other living things. Viruses and prions also cause diseases. Medicines and drugs have been produced to prevent, treat or cure many of these diseases, but by practising hygiene, harmful micro-organisms in everyday life can be reduced to acceptable levels, rather than eliminated. In food preparation, micro-organisms are reduced by cooking, cleanliness, short storage periods or by storing in low temperatures. 

Prokaryotes and eukaryotes

Biologists divide all living things (the "tree of life") into three domains. Two of these domains comprise prokaryotes: microscopic single-celled organisms whose cells have no nucleus. These are the archaea and bacteria. Eukaryotes, the third domain, are organisms made up of cells each of which contains a nucleus enclosed within a membrane. Animals, plants, fungi and protists—that is, all life forms that are not archaea or bacteria—are eukaryotes.

Archaea

Archaea make up a group of single-celled organisms that have no cell nucleus. Until the 1970s, members of this group were classified as bacteria—before scientists concluded that they were distinct from them: archaean cell membranes are made from different substances to those of bacteria.

Archaea were originally thought to be just those organisms capable of living in extreme environments, such as in hot springs or in rocks deep inside the Earth's crust. Later, scientists discovered that archaea are also found in great abundance in all types of habitats, including the air, the soil, the oceans, and inside plants and animals.

Extremophiles—which include some species of bacteria and fungi as well as archaea—divide into a number of groups. Thermophiles thrive in high temperatures, while psychrophiles withstand extremely low temperatures. Halophiles flourish in high salt conditions. A few extremophiles, such as Deinococcus radiodurans, can resist exposure to radiation; they may even be able to survive in space. Their very existence under such extreme conditions increases the likelihood of extraterrestrial life, possibly even in our own Solar System.

Bacteria are found in every habitat on Earth: soil, rock, oceans and snow. Some live on or inside plants and animals—including humans. In fact, our bodies contain countless billions of them: around 10 times our total number of cells. Many of these bacteria are found lining our digestive system.

​​​​​​​Bacteria in the soil play an important role in the cycling of nutrients. Some types of bacteria cause food spoilage and crop damage. Others are useful in the production of fermented foods such as yoghurt and soy sauce. A small number are parasites and act as pathogens, causing diseases in animals and plants.

Protists

Like bacteria, protists are microscopic, single-celled organisms. But unlike bacteria, each protist has its genetic material (DNA) wrapped inside a bag-like membrane to form the nucleus of the cell. Coming in many different shapes and sizes, protists live in a wide variety of moist habitats including fresh water, saltwater and the soil.
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Some protists, such as some kinds of algae, are tiny plants. Called protophyta, they absorb their energy from sunlight and their raw materials for growth from the water around them. Other protists, the protozoa, are microscopic animals. Most types move around and consume food particles such as bacteria. The group includes: flagellates, which move with the help of whip-like structures called flagella; ciliates, which move by using hair-like structures, called cilia, that beat in a regular pattern like oars; amoebae, which move by the use of foot-like structures called pseudopodia. Some protozoa are sessile—they do not move at all.

Fungi

Fungi include micro-organisms such as yeasts and moulds, as well as larger forms such as mushrooms and toadstools, the fleshy, fruiting body of a fungus that grows in the soil or on decaying plants. Fungi cannot make their own food in the way plants do, so they absorb it from their surroundings. They grow anywhere where it is warm and damp, including soil, plants, animal (including human) skin and inside animals' digestive systems. Some fungi reproduce by sending spores, microscopic, dust-like particles, into the air. Wrapped in protective coats and spread on the breeze, in water, on animals or on clothing, spores start to grow if conditions are right in the places where they land.
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Fungal diseases

Some fungal micro-organisms are parasites of plants, where they cause diseases such as rusts, mildews, scabs or canker. Dutch elm disease and chestnut blight are both caused by parasitic fungi. A few kinds of fungi cause diseases in animals, often having devastating effects on populations, for example Ophidiomyces ophiodiicola in snakes and Ophiocordyceps unilateralis in tropical ants. In humans, fungal infections include skin diseases such as athletes’ foot, ringworm and thrush. Some fungi, for example Cryptococcus neoformans (a kind of yeast), can cause life-threatening diseases in humans.

Algae

Although some green algae are classified as protists, others are classified as plants. Algae can grow as single cells, joined together in chains like Spirogyra, or made up of many cells, as in red seaweed. Most algae live in fresh or sea water where they may be either free-floating or attached to the bottom. Some algae grow on rocks, soil or vegetation—wherever there is sufficient moisture. A few algae form partnerships with fungi to form lichens. Some algae even live in the moist fur of animals such as sloths and polar bears.

Like a plant's leaves, algae contain a pigment called chlorophyll which they use to make their own food by photosynthesis. Those algae that appear brown, yellow or red have additional pigments to camouflage their green colour.

Viruses are the smallest of the microbes—but, strictly speaking, they are not micro-organisms at all. This is because they are only alive when they invade living cells, called host cells. Viruses can cause infectious diseases, such as the common cold, flu, chickenpox and measles. Covid-19 is caused by a type of virus called a coronavirus.

A virus is made up of a core of genetic material, either DNA or RNA, surrounded by a protein coat, called a capsid. Sometimes the capsid is covered by a spiky lipid coat called an envelope. The spikes helps the virus to latch on to a host cell and get inside it. To make more viruses—a virus's sole reason for existence—it first attaches itself to the host cell before penetrating it. It then uses the cell’s machinery to replicate (make multiple copies of) its own genetic material. Once replication has been completed, the virus particles leave the host either by budding (taking over parts of the cell membrane to form new viral envelopes) or bursting out of the cell, a process called lysis. Either way, the host cell is destroyed. Once the viruses have escaped from the host cell, they are ready to enter new cells and multiply themselves.

Prions

Prions (proteinaceous infectious particles) are kinds of protein that have an abnormal structure, described by scientists as "misfolded". Prions are not micro-organisms—they are proteins and contain no genetic material—so they are unable to reproduce like other pathogens. But prions cause diseases in animals and humans by transmitting their misfolded shape on to the normal healthy proteins in the brain. Once a misfolded prion enters a healthy person, often in infected food, it converts normal proteins into the misfolded form (it is not yet exactly known how this happens). Prions cause BSE (Bovine Spongiform Encephalopathy, or "mad cow" disease) in cattle, scrapie in sheep and goats, and Creutzfeldt-Jakob disease in humans.