E. coli is so genetically varied that strains share only a fifth of their genes
Escherichia coli is biology's favourite lab workhorse, able to double in about 20 minutes. It is also a species in name only: a typical genome shares just 20 percent of its genes with every other strain, and the dysentery bacteria called Shigella arguably belong inside it.
This rod-shaped bacterium, about two micrometres long, lives in the lower gut of warm-blooded animals, where it makes up roughly 0.1 percent of the microbes. Most strains are harmless or helpful, supplying vitamin K2 and crowding out dangerous invaders. It grows best at 37 degrees Celsius, uses oxygen when available and switches to fermentation or other forms of respiration without it. Its outer membrane shields it from penicillin, and it stains pink in the Gram test.
Its metabolism is famously thrifty. Given a mix of sugars, it eats the one that fuels fastest growth first, glucose before lactose, by switching off genes for the others, a process called catabolite repression. When food is plentiful it starts copying its chromosome again before the last copy is finished, so cell cycles overlap. Researchers have even rewired strains to build all their biomass from carbon dioxide. And though it divides into two seemingly identical cells, the one inheriting the old pole behaves like an ageing parent that keeps producing rejuvenated daughters.
Genes also move sideways between cells through conjugation and virus-mediated transduction. That is how a Shiga toxin gene jumped from Shigella to create the dangerous strain O157:H7. About 18 percent of the common lab strain MG1655's genome arrived this way since its lineage split from Salmonella, roughly 102 million years ago. About 190 serogroups, defined by surface molecules, are known, and host-specific strains let scientists tell whether water was contaminated by humans, other mammals or birds.
Pathogenic types such as EPEC and ETEC spread by the faecal-oral route and trigger food recalls; some strains are frequently lethal to children in developing countries and dangerous for elderly or immunocompromised people. Yet cheap and easy to culture, and studied intensively for over 60 years, E. coli has hosted most recombinant DNA work, with strains K-12 and B the usual lab choices.
Source: Escherichia coli
