system
Given the following post, respond as if you're commenting on Reddit:
human
Did our gut bacteria originally ride into our bodies on the kind of food it helps us digest?

Did bacteria that digests sugars first ride in on something sugary? Did fiber-eating bacteria originally ride in on fiber? Is that anything like the way it works?

Edit: If that is the way it works, does it mean we might be able to engineer symbiotic bacteria to help solve hunger issues?
gpt
That's not really the way it works. If you look at bacteria that are naturally present on food sources and then look at the bacteria in the gut that use that same food source, they are usually quite different. Often this is because of different selective pressures in the two environments. Outside the gut, the bacteria are usually in an aerobic environment, inside it is anaerobic. There will also be differences in pH, salt content, other bacteria that are present that may antagonize them and also the human immune system. Some of the early post-weaning colonizers may enter as you theorize, but they are relatively quickly out-competed by bacteria that are specialized in thriving in the gut. Those get in in a variety of ways, some have environmental reservoirs others are gut specialists coming largely from fecal contamination of the surroundings of a child when their microbiome is developing. The initial pre-weaning colonizers come from mother's vagina or skin depending on delivery mode. What you are describing for solving hunger issues is essentially a synbiotic approach of co-feeding a beneficial bacteria and its substrate. These have by and large not proven to be especially effective, though there may be some transient benefits.