Some people are now getting fæcal transplants from healthy friends in UK.
The donor is screened; the fæces is liquidized, checked for hostile microbes, then swallowed by the deficient patient. Family and friends are suggested as donors to diminish disgust.
Other primate mothers feed their newborn babies a handful of their own fæces which passes-on gut bacteria; I saw a documentary showing this.
Swallowed/injected (systemic) antibiotics can wipe-out our little friends in the gut, vagina (allowing Candida albicans territorial supremacy and causing a thrush bloom) and elsewhere.
We eat pooh-yoghurt containing cultures of gut bacteria.