E. coli

The best-studied bacterium, and mostly your friend/Microscopic Life

Escherichia coli is the workhorse of molecular biology and a mostly harmless resident of your gut. A single cell is about 2 micrometers long, and it can divide every 20 minutes under ideal conditions, meaning one bacterium could theoretically become a billion in about 10 hours. Most strains are beneficial, producing vitamin K and keeping harmful bacteria in check.