Questions and Answers about BIOLOGY

by Ken Miller and Joe Levine


QUESTION: If the E. coli that causes food poisoning is the same E. coli that normally lives in our digestive tract, then why does it cause problems when it gets into the stomach from contaminated food? Also, how do the E. coli in our gut get in there in the first place? [from Mary, a teacher in Washington State]

Problems with E. coli are caused by two things: 1) Location, and Quantity.

E. coli is a common bacterium which is found just about everywhere. Small numbers of E. coli are found in most surface water, even the cleanest, clearest lakes and rivers. This and other bacteria are present on most surfaces in your house, kitchen, and cooking utensils, and therefore in most of the food we eat. That's how a newborn baby gets the E. coli (and other bacteria) that take up residence in the intestines. As you know, those bacteria carry out a number of beneficial functions, helping us to digest many types of food, and are seldom a cause of any harm.

If large numbers of E. coli are present in food or drinking water, however, the waste products and other toxins they normally produce can accumulate, and cause stomach disturbances that may be severe, so it is important to avoid food or water that is excessively contaminated with E. coli. There's something else to worry about, however.

There are many varieties, or "strains," of this bacterium which are quite different from the relatively harmless bacterium found in our intestines. Some of these strains release extremely toxic chemicals that can cause severe food poisoning and, in extreme cases, even death. You can find out more about these bacteria on this web site, from the US Centers for Disease Control:

These deadly bacteria are called E. coli, sure enough, but they are quite different from the "good guys" living in our intestines, as the Q&A link on this page makes clear.

Ken Miller (updated 8/19/09, with special thanks to Rose Daus)

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(A web site developed by Ken Miller and Joe Levine to provide scientific and educational support for teachers and students using our textbooks)