E. coli bacterium, coloured transmission electron micrograph (TEM). Escherichia coli is a rod-shaped bacterium (bacillus), a unicellular organism. This thin slice has cut through the bacterial cell lengthwise, showing the internal structure. Unlike eukaryotic organisms, bacteria do not have a membrane-bound cell nucleus. Instead, their genetic material is located on chromosomal strands (red) in an area called the nucleoid. The bacteria are thus classed as prokaryotes. E. coli is a normal component of the intestinal bacterial flora, but under certain conditions some strains can cause severe infections like gastro-enteritis.