Introducing small amounts of an inert pathogen helps the body prepare antibodies and other defenses against the real thing. On the other hand, there are a myriad of “helpful” bacteria that the body depends on for digestive purposes, and maybe others, for all I know. What is the method by which the body differentiates between “good” and “bad” bacteria? And assuming there is some early-life mechanism, how does the body differentiate after a round of antibiotics have killed off everything and a mixture of useful and annoying lifeforms have been reintroduced?
“Helpful” bacteria can be just as dangerous to you as “harmful” bacteria. The bacteria that live in your gut can cause infection or serious damage to immunocompromised people, or if they get in the blood stream. There was an E. coli outbreak last month, for instance, that made more than 26 people sick, but you have E. coli in your intestines right now, and aren’t sick from it.
It doesn’t. Antibodies and the rest of the immune system occupy the circulation system. Any bacteria that get into it are assumed to be enemies by the immune system and eliminated. That includes any “good” bacteria, since they can cause harm in there.
“Good” bacteria live in areas outside the circulation system, mainly the GI tract, but also outer surface of the body (pores on the skin, various orifices, etc.). Not all bacteria that live in these areas are necessarily good, of course. Helicobacter pylori comes to mind as an example of a non-good bacteria that lives in the stomach of a rather large percentage of humans.
The bacteria basically fight it out amongst themselves. Most “good” bacteria have evolved to live in specific areas and so when they get into those areas, they can out-compete pretty much anything else occupying their spot.
The body may help somewhat in this process, but I think its main contribution is just providing the specific environments that the good bacteria are evolved to inhabit.
So does this mean that intravenous antibiotics won’t cause the stomach upset of oral antibiotics? Maybe I can get my doctor to give me a shot instead of pills that cause so much gastric distress as to knock me out for a week. I’m always pounding the acidophilus to try and stave that off, and I find I still can’t take the entire round of antibiotics. And I know that I risk reinfection.
The E. Coli that make people sick are a particular variety of E. Coli, namely E. Coli 0157:H7. That’s why the E. Coli in your gut don’t (normally) make you sick - they aren’t the nasty kind.