E. coli bacterium, animation. E. coli bacteria are a normal part of the intestinal flora in humans and other animals, where they aid digestion. However, some strains can produce a toxin that leads to severe illness, or even death. Normal strains can also produce infections in those with weakened immune systems. A typical bacterium is covered by an external capsule (with 'hairs' called fimbriae), and has a rigid cell wall, and an inner cytoplasmic membrane. Some bacteria also have appendages called flagella that they use for locomotion.