Faecal bacteria. Scanning electron micrograph (SEM) of various bacteria found in a sample of human faeces. At least 50 per cent of human faeces is made up of bacteria shed from the gut. Many of these bacteria are a normal part of the flora found in the intestines and are beneficial to digestion. However, some are pathogenic, such as Salmonella enterica and certain strains of Escherichia coli, which can cause foodborne illnesses. Magnification: x16,700 when printed 10 centimetres wide. Credit: EYE OF SCIENCE/SCIENCE PHOTO LIBRARY.