Escherichia coli. Artwork of a section through the bacterium Escherichia coli (E. coli). The cell wall (green, lower left to upper right) contains two membrane layers studded with proteins. A flagellar motor and flagellum is shown at top left. The cytoplasm contains ribosomes (purple), tRNA molecules (maroon), enzymes (blue) and mRNA (white). The lower right portion of the image shows the cell nucleus (yellow and orange). E. coli is a rod-shaped bacterium (bacillus). It is a normal component of intestinal bacterial flora, but under certain conditions can cause gastroenteritis and other infections.