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A microorganism is any form of life that is too small to be seen without a microscope. Also called microbes, these tiny organisms include bacteria, protozoa, single-celled algae, and fungi as well as viruses. Microorganisms are nearly everywhere and are essential to the production of certain medicines, foods, and drinks. They play a key role in nature's oxygen, carbon, and nitrogen cycles, but can also be harmful to humans.

The study of microorganisms is called microbiology. It was founded in the seventeenth century after the microscope was invented. Not until the Dutch naturalist Anton van Leeuwenhoek (1632–1723) saw what he called "little animalcules" (or little animals) with his own microscope in 1673, did science know of the existence of a subvisible world that was teeming with life. Despite this major discovery, no one knew where these microscopic forms of life came from. Most believed that the life forms simply sprang out of rotten wheat or from the soil. This incorrect notion of spontaneous generation was held until 1861 when the French chemist Louis Pasteur (1822–1895) was able to prove that spontaneous generation does not occur and that the air itself is full of microorganisms. Pasteur also discovered that fermentation was caused by microorganisms. (Fermentation is a process in which cells break down sugar or starch into carbon dioxide, ether, alcohol or lactic acid.)

Pasteur went on to make many other contributions and is considered the founder of the science of microbiology. He discovered that microorganisms are present in nonliving matter as well as in the air, and that some of these tiny organisms caused disease. He also showed how microorganisms could be killed by heat and how they could be manipulated for use in vaccines. In 1876, the German physician Robert Koch (1843–1910) was able to demonstrate that a particular bacterium could cause a particular disease. He isolated the bacteria that produced the cattle disease anthrax, and he discovered the bacteria that caused tuberculosis and cholera. With the discovery of viruses just before the turn of the century, it was realized that although viruses are not living organisms (since they cannot grow and reproduce on their own), they are even smaller than bacteria and are physically microorganisms.

There are several types of microorganisms. Protists are a group of single-celled plantlike or animal-like organisms that have complex or eukaryotic cells. This means that their genetic material is contained in a nucleus that is bound by a membrane. Protists (kingdom Protista) are far more diverse than plants or animals and obtain their food by simply absorbing it from their environment since most live in water. They also have different means of getting about, such as flagella (tails) and cilia (hairs). The well-known euglena is a protist, as are all single-celled algae and the many differently shaped diatoms that form glasslike shells. Bacteria are another group of microorganisms (too small to be seen with the naked eye), that belong to the kingdom Monera instead of Protista. Among the most abundant life forms on Earth, bacteria are single-celled organisms that have a cell wall but no nucleus (meaning that they are prokaryotic and not eukaryotic). Some bacteria feed on dead matter and play an important role in recycling nutrients, while others cause disease. Fungi are microorganisms that make up their own kingdom (Fungi). The smallest fungi can be microscopic and single-celled. Like protists, fungi also absorb their food from their environment. Finally, viruses are microorganisms by virtue of their size. They are not considered living organisms because they are parasites and can reproduce only by taking over their host's cellular machinery. They are the tiniest of all the microorganisms and can only be seen with an electron microscope.

Microorganisms play a major role in the environment. As recyclers, they break down many substances into usable forms for plants and animals. Without microorganisms the world would be full of waste. Microorganisms are also crucial to several key industries. For example, the production of antibiotics, vaccines, beer, cheese, wine, and bread would be impossible without microorganisms. Although sometimes beneficial, microorganisms also can cause diseases. Some of these diseases are less dangerous than in the past because scientists have been able to develop cures. However, other diseases continue to be harmful as microorganisms adapt and mutate in response to treatment.

Knowledge of microorganisms has allowed biologists to use them as experimental models to study the chemical processes of more complex organisms. Microorganisms have been crucial to our knowledge of deoxyribonucleic acid (DNA) and have proven to be an essential tool of genetic engineering in which biologists experiment with the genetic code of living organisms. Altogether, microorganisms are not only essential to life on Earth, but also help scientists solve problems in medicine, agriculture, industry, and the environment.