 E. coli belongs to a group of bacteria informally known as coliforms that are found in the gastrointestinal tract of warm @-@ blooded animals . E. coli normally colonizes an infant 's gastrointestinal tract within 40 hours of birth , arriving with food or water or from the individuals handling the child . In the bowel , E. coli adheres to the mucus of the large intestine . It is the primary facultative <unk> of the human gastrointestinal tract . ( Facultative anaerobes are organisms that can grow in either the presence or absence of oxygen . ) As long as these bacteria do not acquire genetic elements encoding for virulence factors , they remain benign commensals . 
