It is. Fortunately, bacteria are everywhere, and people are messy and sloppy. One way or another, everyone develops a gut biome.

It's just that the best biome comes from your mother, and establishes early. There is speculation that some issues - including obesity - are strongly related to the specific gut biome people gain in early childhood.

Some morbidly obese people might - I stress might, it hasn't been solidly proved yet - could be victims of, well, not enough poop all over them when they were born. They picked up a gut biome from the environment after, and it may not do the right job - goes the theory.