E. coli bacteria in small intestine. Fluorescence light micrograph of enteropathogenic Escherichia coli (EPEC) bacteria (red-green dots) on human small intestine epithelium. This view looks down on the intestinal surface. This image highlights the difference in size between bacteria and eukaryotic cells. E. coli bacteria attach to the surface of human intestinal cells. During this process, they sequester the actin (green) in the host cell to form actin-rich pedestals. Once attached, the bacteria cause the tight junctions (blue) between the cells to break down, leading to the leakage of water into the intestine, resulting in diarrhoea.