E. coli and Shigella sp. bacteria. Fluorescence confocal light micrograph of Escherichia coli and Shigella sp. bacteria (blue) in human Caco-2 cells (green), a cultured cell line derived from a colorectal carcinoma. The cell nuclei are red. These are motile bacteria that are capable of entering intestinal cells (enteroinvasive) and causing disease. Shigella sp. bacteria cause dysentery, which can vary in severity from a mild attack of diarrhoea to an acute infection. Enteroinvasive E. coli cause severe diarrhoea that can be fatal, especially in the very young or elderly.