E. coli
A gram negative, rod shaped bacterium that is mostly harmless, but some strains can cause serious illness [1]. Commonly used in genetic engineering to mass produce proteins by inserting recombinant plasmids (e.g. to produce insulin). It is also used as a model organism because it can be easily manipulated, has a short generation time and its genome has been sequenced.
See Escherichia coli for more details.