Escherichia coli bacteria by scanning electron microscopy (SEM). E. coli are present in the intestinal contents of animals and are thus also called the intestinal rods. They are used as indicators of contamination with fecal material. In the past, some of these originally harmless bacteria developed toxins and became a serious health hazard to humans and animals. Enhanced SEM: Image width: 9.1 micrometers; magnification: 10,990x if the image is printed 10 cm wide.