Bacterial phagocytosis. Light microscopy footage of a phagocytic cell engulfing and destroying two Escherichia coli (E. coli) bacteria (centre). 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. Several cells of the human immune system are phagocytic, seeking out and destroying invading bacteria.