Faecal bacteria. Scanning electron micrograph (SEM) of bacteria cultured from a sample of human faeces. A single very long bacterium is present. 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: x30000 when printed 10 centimetres wide.