Escherichia coli bacterium, illustration. E. coli is a rod- shaped bacterium (bacillus). Its cell membrane is covered in fine filaments called pili or fimbriae. Hair-like structures called flagella at the rear of each bacterium provide propulsion to make it move. E. coli is a normal component of the intestinal bacterial flora, but under certain conditions some strains can cause severe infections such as gastroenteritis.