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What are microbes?
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Salmonella Digitally colorized scanning electron micrograph of Salmonella (red) bacteria in the process of invading an immune cell (yellow).

What are microbes?

Written by
Kara Rogers
Kara Rogers is the senior editor of biomedical sciences at Encyclopædia Britannica, where she oversees a range of content from medicine and genetics to microorganisms. She joined Britannica in 2006 and...
Kara Rogers
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Britannica Editors
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
Britannica Editors
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Microbes, also called microorganisms, are living agents that are too small to be seen with the unaided eye. They include bacteria, archaea, many fungi, protozoa, and microscopic algae. Viruses are generally not considered to be living organisms, since they cannot reproduce independently; however, they are often studied alongside microbes because of their incredibly small size and their interactions with living cells.

Microbes are among the oldest and most abundant forms of life on Earth. They inhabit virtually every environment, from oceans, soils, and freshwater lakes to hot springs, deep-sea vents, polar ice, and even the atmosphere. Many microbes also live on and inside plants and animals, including humans. The human body alone hosts trillions of microorganisms—collectively known as the microbiome—that aid digestion, produce vitamins, help train the immune system, and prevent harmful microbes from becoming established.

Most microbes are harmless or even beneficial. Microorganisms such as cyanobacteria and many algae generate a substantial portion of Earth’s oxygen through photosynthesis. Other microbes recycle nutrients by decomposing dead plants and animals, fix atmospheric nitrogen into forms that plants can use, and form essential partnerships with roots, insects, and other organisms. Humans also harness microbes to produce foods such as yogurt, cheese, bread, and soy sauce, as well as fermented beverages and certain medicines, including antibiotics, vaccines, and insulin.

Disease-causing microbes, known as pathogens, include certain bacteria, fungi, protozoa, and viruses. They are responsible for illnesses ranging from pharyngitis (strep throat) and tuberculosis to malaria, influenza, and COVID-19. However, pathogenic microbes represent only a small fraction of microbial diversity. Advances in microscopy, DNA sequencing, and genomics continue to reveal the enormous diversity of the microbial world and its central role in ecosystems, agriculture, biotechnology, and human health.

Kara Rogers