|MadSci Network: Microbiology|
Most E. coli are "normal flora" bacteria. These bacteria live in our intestines at about 100,000,000 bacteria per gram of intestinal matter - - so there are a lot in our intestines. Before the early 1960's no one had ever heard of a "bad" E. coli. But, from evolution and horizonal transfer of DNA, several E. coli "picked up" toxin genes. These toxin genes caused the E. coli to become pathogens. These bacteria are still E. coli, but they are now armed with toxin genes that can damage tissues, cause diarrhea, and even kill people.
The way that these E. coli pick up toxin genes is by being infected with a virus. There are bacterial viruses (they do not affect humans) that can give these E. coli the DNA for toxin genes. These genes make the E. coli toxigenic. Now there are several types of Toxigenic E. coli - - STEC, ETEC, EHEC, EAEC, and others.
I hope this answers your question.
Richard J. Obiso, Jr., Ph.D.
East Tennessee State University
Try the links in the MadSci Library for more information on Microbiology.