Escherichia coli
As E. coli you would wake up in a warm, dark, crowded intestine and spend your life splitting in two. Most of your relatives are harmless roommates; a few are the reason people wash their hands.
A rod in the gut
Escherichia coli is a rod-shaped bacterium about 2 micrometres long – you could line up 500 of them across a millimetre. It lives in the lower intestine of humans and other warm-blooded animals, where it is a minority member of a community dominated by oxygen-hating species. E. coli is more flexible: it can breathe oxygen when there is some and ferment sugars when there is none, which makes it one of the first colonisers of a newborn’s gut.
You would move with whip-like flagella, steering by a simple rule: swim straight while conditions improve, tumble randomly when they get worse. It is a surprisingly effective way to find food without a brain.
Good neighbour, occasional villain
Most strains are harmless or even useful, producing vitamin K2 and crowding out more dangerous microbes. A minority have picked up extra genes that make them pathogenic. Shiga toxin–producing strains such as O157:H7 can cause bloody diarrhoea, usually after eating undercooked minced beef, raw milk or contaminated vegetables; other strains are the most common cause of urinary tract infections. Thorough cooking, clean water and handwashing prevent most cases.
The lab’s workhorse
Your descendants would have a remarkable career. Strain K-12, isolated from a patient in 1922, became the standard organism of molecular biology: it helped reveal how genes are switched on and off and how DNA is copied. Since 1982, engineered E. coli has produced human insulin for people with diabetes. In good conditions you divide roughly every 20 minutes, so a single cell could in theory found a colony of billions overnight – in practice food runs out long before that, and most cells end life starved, eaten by protists or flushed out of the body.
