Availability of lactating goats or dogs. Ability to make and isolate anti-biotic resistant mutants. But as even young scientists know, the normal is like the control. Only when something goes wrong with the normal does disease step in. Alas, the medical community knows precious little about the normal.
Medical textbooks state plainly that in the womb, the fetus is sterile and has no bacteria.
And that the intestinal bacteria are gained during the birthing process by a mechanism called reflux, where fluids from the outside world rise up the birth canal to inoculate the baby. Babies born via caesarian should thus not have any bacteria. At the instant that almost all babies are born, they already have about a hundred trillion bacteria in their intestines. That number would look like peanut butter. Many nursing women contract mastitis, which is a bacterial infection of their milk ducts by some environmental bacterium.