Escherichia coli bacteria in blood, computer illustration. E. coli is a gram-negative motile bacterium with peritrichous flagella that lives in the intestines as part of its normal microflora. However, some strains can cause diarrhoea and infections outside of the intestine, such as bacteraemia (bacteria in blood), peritonitis, cystitis, wound infections, nephritis, gynaecological infections, and meningitis in newborns.