Whence fecal E. coli (et al.) if swallowing it is dangerous?

Question

I don't know much about medicine, and I know even less about microbiology, but I understand that there are organisms in the lower gastrointestinal tract (and in feces) of a human, like Escherichia coli, that can cause dangerous disease if swallowed (or injected intravenously) by a human. So how did they get to the lower gastrointestinal tract in the first place? I assume either (or some combination of)

• its prevalence in feces is less than I'm led to believe, so my question as to how it got there doesn't arise;
• the percentage among cases of swallowed E. coli of those that lead to disease is low; or
• E. coli-caused disease is more common than I'm lead to believe.

Does anyone know which of these is true? Or is it something else? (Perhaps they enter the gastrointestinal tract from below and not via swallowing?)

Answer 1

E. coli is mostly harmless; only a few strains are harmful.

I don't believe the route by which gut biota is established has been entirely established for any species but, for example, koala feed their faeces to their offspring to help them establish biota capable of digesting eucalyptus. It seems that a small proportion of ingested bacteria somehow survive passage through the stomach to the gut and then begin multiplying and colonising the intestines.

It's also important to realise that just because an organism is sometimes harmful doesn't mean it always is. There are many microorganisms that are pretty much ubiquitously found on or in your body which are capable of causing disease but only do so very rarely; usually when the body's defences have already been compromised (e.g. Pseudomonas aeruginosa, Candida albicans aka thrush, and so on).

Answer 2

There are many varieties of E. coli, with differences in their chromosomal DNA, different plasmids (independent loops of DNA that replicate autonomously inside the host cell and can be transferred between bacteria), or both. Some are harmless, some beneficial, and some deadly. E. coli of various kinds are found in the feces of cows, dogs, people, and more; but are also found in the environment. Of course, even normally harmless bacteria can be harmful if they are in the wrong place in the body (e.g., in the blood) or if the body's microbial balance is out of whack.

Your second thought is correct:

the percentage among cases of swallowed E. coli of those that lead to disease is low

E. coli is found practically everywhere in homes, in concentrations ranging from very sparse to very dense. A good eye-opener is [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3709336/], which makes it very clear that children (and adults) are exposed to E. coli constantly. We hardly notice until a strain comes along that happens to make us seriously ill.