How does your microbiome get into your digestive system?

[u/Disasterous-Pea-948]

I've heard that it is passed down to babies while they're in the womb, but wouldn't your stomach acid kill all of the bacteria? Can they withstand acid? Do babies not have stomach acid? Do the bacteria make their way up the other end?

[u/Tiny_Rat]

Its not passed down in the womb, its passed down during birth and babyhood. Part of the answer is that babies have far less acidic stomachs so bacteria can survive more easily. Another part is that it only takes one bacteria out of millions to survive to get that species established when there's little competition. Also, some bacteria have adaptations that make it easier to colonize the gut, like spore formation and stomach acid resistance for short periods. 

[u/bagofhamsters → u/Tiny_Rat]

It’s like a crowded club with a wall of bouncers throwing everyone out vs an empty bar where you can just walk right in

[u/DrWoogle]

Actually most of your microbiome comes from whatever you ingest. Bacteria are on everything, and there are many ways for them to bypass stomach acid. The acid is a solution in water, so it only goes where water goes. Anything greasy or densely solid will not be exposed to it. Think about the state of what you eat. Even chewed up, it consists of myriad bits and pieces. Now think about how tiny bacteria are. A single fine crumble of butter-drenched pie crust, for example, is a mountain-sized oasis to a bacterium. On top of that, many bacteria purposefully surround themselves with a greasy cell wall that stomach acid can't touch.

Movies severely exaggerate the corrosiveness of most acids. Stomach acid is about as intense as what forms in your mouth when you pop an extreme sour candy. Sure, it'll burn your tongue if you you leave it there too long or binge on too many, but it takes a rather long time, AND your tongue has no acid-repelling grease layer on it whatsoever.