Problem:

Clearly, a zygote does not harbor any microbes. As it develops, and the alimentary canal tissue is differentiated, I logically assume that there is still no microbial activity in the fetus's gut. I'm thus also assuming that the commencement of microbial activity in the intestines occurs after birth, but when, and how?

Do newborn babies just happen to ingest only certain e.g., E.coli strains? Is the gut a selective environment in which only certain E.coli strains develop?

Are there any mechanisms to ensure that harmful bacteria do not initially reside in the gut? Is a newborn's digestive tract particularly unstable and ineffective until the proper microbes are established?

Please give the full description about the mechanism of harmful bacteria.