Dry pink colonies of E.coli in MacConkey Agar Escherichia coli (abbreviated as E.coli) is a Gram- negative, lactose fermenters, facultative anaerobic, rod- shaped coliform bacterium of the genus Escherichia that is commonly found in the environment, foods and lower intestines of people and animals. E.coli are a large and diverse group of bacteria. Although most strains of E.coli are harmless, others can make you sick. Some kinds of E.coli can cause diarrhea, while others cause urinary tract infections, respiratory illness and pneumonia, and other illnesses. Antibiotics can effectively treat E.coli infections.