Escherichia coli, usually shortened to E. coli, is a rod-shaped bacterium that lives in the intestines of people and animals. A typical E. coli cell is about 2 micrometers long and about 1 micrometer wide. A micrometer is a millionth of a meter. Most kinds of E. coli are harmless, but some can make people sick.

Size

E. coli is shaped like a tiny cylinder with rounded ends. Biologists use 2 micrometers long and 1 micrometer across as a rule of thumb, based on many years of measurements with light and electron microscopes. A cell that size has a volume of about 1 cubic micrometer and a mass of about 1 picogram, a trillionth of a gram.

The size depends on how fast the bacteria are growing. Cells given richer food grow faster and are larger. By dry weight, cells that divide every 24 minutes are almost six times as heavy as cells that divide every 100 minutes. In one electron microscope study, cells that divided about once an hour were 1.94–2.72 micrometers long.

Each cell also changes length during its life. It grows longer, then pinches in two at the middle. On average, each new cell starts at half the length its parent reached before dividing.

Size comparisons

About 40–50 E. coli cells laid end to end would span the width of a human hair, which is 80–100 micrometers wide.
About four E. coli cells laid end to end would span a red blood cell, which is about 7.5 micrometers across.
If an E. coli cell were enlarged to the height of a person 1.7 meters (5 feet 7 inches) tall, a human hair would be 68–85 meters (220–280 feet) wide.
Particles of the giant virus Pithovirus sibericum, about 1.5 micrometers long, are about three-quarters the length of an E. coli cell.

Where E. coli lives

E. coli is found in many places, including the environment, food, water and the intestines of people and animals. Most kinds are a normal part of a healthy intestine. There, they help digest food, produce vitamins and protect against harmful germs.

Some kinds, however, cause illnesses such as diarrhea, urinary tract infections and pneumonia. People can be infected by swallowing these bacteria in contaminated food or water, or through contact with animals or other people.

How E. coli grows and moves

E. coli reproduces by dividing in two, a process called binary fission. In the best laboratory conditions, a population can double in as little as 20 minutes. At that rate, one cell could in theory become more than a billion in 10 hours.

The cell swims using flagella, stiff spiral strands that stick out all over its surface and spin like propellers. When the flagella turn one way, they bundle together and push the cell forward. When they turn the other way, they spread apart and the cell tumbles, turning to face a new direction.

Sources

How big is an E. coli cell and what is its mass?. In Cell Biology by the Numbers. Ron Milo and Rob Phillips, Garland Science.
Cell length (BNID 100001). BioNumbers, Harvard Medical School, citing Grossman, Ron and Woldringh, Journal of Bacteriology, 1982.
Cell-size control and homeostasis in bacteria. Taheri-Araghi et al., Current Biology, 2015.
About E. coli Infection. Centers for Disease Control and Prevention, 2024.
How Microbes Grow. In Microbiology. OpenStax, 2016.
Unique Characteristics of Prokaryotic Cells. In Microbiology. OpenStax, 2016.
How big is a human cell?. In Cell Biology by the Numbers. Ron Milo and Rob Phillips, Garland Science.
Thirty-thousand-year-old distant relative of giant icosahedral DNA viruses with a pandoravirus morphology. Legendre et al., Proceedings of the National Academy of Sciences, 2014.
Just How Small Is "Nano"?. National Nanotechnology Initiative.
SI prefixes. International Bureau of Weights and Measures (BIPM).

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