E. coli is the abbreviated name of the bacterium in the Family Enterobacteriaceae named Escherichia (Genus) coli (Species). Dave Graham in the Department of Microbiology, University of Illinois at Urbana/Champaign, recently pointed me to information gleaned from G.W. Tannock's book, Normal Microflora,1995, Chapman & Hall, which reveals that approximately 0.1% of the total bacteria within an adult's intestines (on a Western diet) is represented by E. coli. Although, in a newborn infant's intestines E. coli, along with lactobacilli and enterococci represent the most abundant bacterial flora.
In fact, it is for this reason that the organisms which happily inhabit the intestinal tract as normal flora are named enteric bacteria. The Family to which E. coli belongs (Enterobacteriaceae, is named what it is - because of the Greek word enterikos - which pertains to the intestine. The name Escherichia comes from the name of the person Escherich, who in 1885 first isolated and characterized this bacterium.

I thought E.coli bacteria were OK....

You are correct, for the most part. The presence of E. coli and other kinds of bacteria within our intestines is necessary for us to develop and operate properly, and for us to remain healthy - E. coli, along with other species of bacteria, provide us with many necessary vitamins for example. The bacteria make the vitamins, and we gladly absorb them. We pretty much depend upon E. coli in our intestines for our source of Vitamin K and B-complex vitamins.
The fetus of any animal is completely sterile. Immediately after birth however, the newborn acquires all kinds of different bacteria which live symbiotically (we help them to live, and they help us to live) with the newborn and throughout the individual's life. From the day we are born, we are _never_ without bacteria. However, the helpful bacteria like these are located "only" in regions of our body directly exposed to the environment, e.g., our intestines, upper and lower respiratory tract, etc... and never within our bloodstream or the tissues inside our body. Sounds weird, but, it's true - billions of these little critters chugging away, making things we need, helping to digest our food, etc., -- very important to us and most of them are exceedingly kind to us - except when they become teenagers and enter the throes of puberty (just kidding). Indeed, animals who are born and raised "germ free", are really wimpy.... they have thin intestinal walls, puny heart output, and require lots of vitamin supplements just to stay alive.

So, when are E.coli bacteria bad for us?

Truly, you have billions of "friends" that you never knew you had. So, "What's the big deal about E. coli?" you ask with a puzzled expression on your face. Well, now I need to talk about the "bad guys." Bacteria are somewhat like humans in that certain individual humans are not very nice - and we know that some individual humans can be downright dangerous. Of course _all_ humans belong to the Genus/Species Homo sapiens, and _all_ E. coli belong to, well, Escherichia coli. So, as there exist individual humans, so too can different individuals exist among E. coli bacteria - we call such individuals a different "strain" of bacteria within a given species. Some of these different strains of bacteria (there may be several within a given species) can be harmful to us. Each of us - given the assumption that a human is reading this information - is sort of a strain of the human species, sapiens. We are different because we are genetically different, e.g., unless we are one member of an identical twin pair, the combination of genes each of us possess is different from every other human on the face of the earth - or arm of the earth for that matter. If you think about it a little bit, there are only about 5-billion (is that about right?) humans who are alive on the earth - there are probably that many bacteria in your intestine alone - don't worry - they don't take up much room.... So, it is possible for us to acquire an individual strain of E. coli which mixes with the other E. coli in our intestines. Now, since an individual strain of E. coli may exist, this situation means that this particular strain of E. coli is genetically different than the vast majority of E. coli in our intestines; otherwise, it would not be a different strain of this organism. If this E. coli strain happens to have genetic information for producing something harmful to us, then, we may be in trouble.

OK; who "is" the bad guy?

The rare strain of E. coli that is getting a lot of "press" lately because it is indeed a bad bug, is E. coli O157:H7, a member of the EHEC - enterohemorrhagic E. coli group. Enterohemorrhagic means an intestinally- related (here we are at the Greek word enterikos again) organism which causes hemorrhaging - and therefore, loss of blood. The image shown below is a picture of an electron microscope visualization (an electron micrograph) of O157:H7 (compliments of David Graham, University of Illinois at Urbana/Champaign).

How then do we "pick up" this organism?

Basically, here is the problem: E.coli bacteria are everywhere in the environment. But, since they are such a common occupant of all animals, anytime we eat something, drink something, or touch our hands to something that has been either a part of or has been near where animals are, there is always the potential to ingest these bacteria - is a good reason for washing your hands now and then, huh.