What is the approximate length of a typical Escherichia coli bacterium?

A typical Escherichia coli bacterium is approximately 2 micrometers long.

E. coli cells are usually rod-shaped, measuring about 2.0 micrometers in length and roughly 0.25–1.0 micrometers in diameter. That makes one cell far too small to see without a microscope, although millions can fit on the head of a pin. The species is a Gram-negative, non-spore-forming, facultative anaerobic bacterium.

E. coli is best known for living in the intestines of humans and many other animals. Most strains are harmless and form part of the normal gut community, but some produce toxins or invade tissues and can cause foodborne illness or urinary-tract infections. Its rapid growth and well-studied genetics have also made it one of biology’s most important laboratory organisms.

A common mix-up is confusing the bacterium’s length with its diameter: 2 micrometers describes its long axis, not its width. Its size can vary somewhat with strain and growth conditions.