E.coli is a Gram-negative, facultative anaerobic organism, Bacillus (shape) rod-shaped, spore forming bacterium that is commonly found in the lower gastrointestinal tract of warm-blooded organisms (endotherms). Morphologically cells are typically rod-shaped, and are about 2.0 microns (μm) long and 0.5 μm in diameter, with a cell volume of 0.6– 0.7 (μm)3.
The genera Escherichia and Salmonella diverged around 102 million years ago (credibility interval: 57–176 mya), which coincides with the divergence of their hosts: the former being found in mammals and the latter in birds and reptiles. This was followed by a split of the escherichian ancestor into five species (E. albertii, E. coli, E. fergusonii, E. hermannii and E. vulneris.).