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Bacteria 101

What are bacteria? 

Bacteria have been around for a very long time, as they are the oldest known forms of life on Earth. The earliest fossils of prokaryotes (a group of organisms that bacteria are part of) dates to over 3.5 x 10years ago.

Bacteria have evolved into a huge variety in order to adapt to a multitude of different environments, including the human body, artic ices, hot-springs and even the bottom of the ocean.

Bacteria Phylogenetic Tree - Where To Find?

(bacterial tree of life derived from

How large are bacteria?

Bacteria are single-celled organisms. Bacterial cells are much smaller than human cells. Bacterial cells can measure from about 1 to 10 μm long. Most of them are only about 1 to 2 μm in diameter. 

A bar along the bottom indicates size of various objects. At the far right is a from egg at approximately 1 mm. To the left are a human egg and a pollen grain at approximately 0.1 mm. Next are a standard plant and animal cell which range from 10 – 100 µm. Next is a red blood cell at just under 10 µm. Next are a mitochondrion and bacterial cell at approximately 1 µm. Next is a smallpox virus at approximately 500 nm. Next is a flu virus at approximately 100 nm. Next is a polio virus at approximately 50 nm. Next are proteins which range from 5-10 nm. Next are lipids which range from 2-5 nm. Next is C60 (fullerene molecule) which is approximately 1 nm. Finally, atoms are approximately 0.1 nm. Light microscopes can be used to view items larger than 100 nm (the size of a flu virus). Electron microscopes are useful for materials from 1.5 nm (larger than an atom) to 1 µm (the size of many bacteria).

(image credit from