E. coli bacterial culture (pink lumps) being grown in a petri dish on a nutrient medium (red). There are several different strains of E. coli (Escherichia coli) bacteria. Some strains are non-pathogenic and are found in human intestines. Other strains cause food poisoning with bloody diarrhoea. These strains can be ingested from contaminated beef and milk. Urogenital infections are also possible. This culture was grown using MacConkey agar, which is used to identify lactose- fermenting enteric pathogens (such as E. coli). The fermentation of lactose turns the medium red as pH indicators respond to the acids formed.