Escherichia coli

E. coli is a rod-shaped, Gram-negative bacterium that can grow with or without oxygen (facultative anaerobe). It colonizes the lower intestine of humans and other animals within hours of birth, forming a mutualistic relationship by producing vitamins and outcompeting pathogens.¹ In the environment, it can survive in water and soil, though it is not a natural resident there; its presence is used as an indicator of fecal contamination.⁴ The bacterium is remarkably versatile, capable of switching between aerobic and anaerobic metabolism, and can utilize a wide range of carbon sources, including sugars, amino acids, and fatty acids.³

E. coli K-12 was one of the first organisms to have its genome fully sequenced (1997), revealing a circular chromosome of about 4.6 million base pairs.³ Its genetic tractability—via plasmids, phage transduction, and conjugation—made it the workhorse of molecular biology, leading to the development of recombinant DNA technology and the production of human insulin. The bacterium's rapid growth and simple genetics have also made it a model for studying gene regulation, metabolism, and evolution, with thousands of mutant strains available for research.⁵

While most E. coli are harmless, certain strains have acquired virulence factors that cause disease. Enterotoxigenic E. coli (ETEC) is a leading cause of traveler's diarrhea, while enteropathogenic (EPEC) and enterohemorrhagic (EHEC) strains cause severe intestinal infections; EHEC, such as O157:H7, produces Shiga toxin and can lead to hemolytic-uremic syndrome.² Uropathogenic E. coli (UPEC) is the most common cause of urinary tract infections, accounting for 80–90% of cases. Antibiotic resistance is a growing concern, with multidrug-resistant strains emerging, complicating treatment.

Beyond its fame as a lab organism, E. coli has surprising roles. It is used in synthetic biology to produce biofuels, pharmaceuticals, and even spider silk proteins. Some strains are probiotic, such as E. coli Nissle 1917, used to treat gastrointestinal disorders.⁵ In the lab, the bacterium has been engineered to produce electricity in microbial fuel cells and to sense environmental pollutants.⁴ Historically, the discovery of the lac operon by Jacob and Monod in 1961 laid the foundation for understanding gene regulation, a concept now central to all biology.³ Additionally, E. coli has been used in astrobiology experiments to test survival in space conditions.

Gram-negative

Bacteria that do not retain crystal violet stain in Gram staining, having a thin peptidoglycan layer and an outer membrane.

Facultative anaerobe

An organism that can grow with or without oxygen.

Commensal

An organism that lives in a relationship where one benefits and the other is neither helped nor harmed.

Shiga toxin

A potent toxin produced by certain E. coli strains that inhibits protein synthesis in host cells.

Hemolytic-uremic syndrome

A condition characterized by hemolytic anemia, acute kidney failure, and low platelet count, often caused by EHEC.