E. coli bacterium in small intestine. Fluorescence light micrograph of an enteropathogenic Escherichia coli (EPEC) bacterium (upper left, red-green dot) on human small intestine epithelium. The intestinal columnar epithelial cells extend from the blue luminal surface (top) to the bottom of the image and beyond. The large red ovals are the nuclei of epithelial cells. 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.