Microorganism

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A cluster of Escherichia coli bacteria magnified 10,000 times.
A cluster of Escherichia coli bacteria magnified 10,000 times.

A microorganism or microbe is a living thing (organism) that is too small to be visible to the human eye (microscopic). The study of microorganisms is called microbiology. Microorganisms can be bacteria, fungi, archaea or protists.

Viruses and prions are not called microorganism because they are classified as non-living. Micro-organisms are often described as single-celled, or unicellular organisms, but there are unicellular protists that are visible to the human eye, and some multicellular species are microscopic.

Microorganisms live almost everywhere on earth where there is liquid water, including hot springs on the ocean floor and deep inside rocks within the earth's crust.

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