A student complains that it makes no sense to worry about coliform bacteria in water, since we naturally possess harmless coliforms in our intestines anyway. Why do regulatory agencies worry about coliform bacteria in water supplies, then?

Some types of coliform bacteria can cause death if ingested.

Explanation:

We have coliform bacteria as part of our bacterial flora. Most coliform bacteria are not harmful. However, there are some strains such as E. coli (Escherichia coli) that can cause serious diseases. Hemolytic Uremic Syndrome is one of them. Bacteria can be found in contaminated water, poorly washed vegetables and raw meats.