Where did all these creatures come from!?!?

So, we've established the basic principle that human beings and other animals (with rare exception) are basically walking petri dishes, and that this is a necessary/good thing. When does this all happen, though? Are we born with microbial zoos in our intestines? Does a newborn's mouth have the same variety in bacterial species as that of a five year old?

Not quite. The womb is a pretty sterile place, so fetuses aren't generally privy to the many wonders of having a microbiota. The magic happens at birth: whether C-section or vaginal delivery, a newborn's first exposure to the world outside its mother is also its body's first encounter with microbes. Birth is a pretty important event in microbiota development. Prego ladies' vagina forest populations actually evolve during the course of pregnancy to include milk-digesting species of bacteria. On the way out, newborns ingest some of those microbes and are colonized, giving them a leg up in the whole digesting lactose game.

sweet, sweet victory.

Breastfeeding kids also have a head start in the grand scheme of colonization, as their intestines become home to Bifidobacterium faster than their formula-quaffing buddies. C-section kids tend to have it the worst. First off, that's how scientists breed gnobiotic mice, or mice with limited or no microbiota... so right away, it's kind of a free-for-all in terms of colonization. Instead of harboring vaginal microbiota-associated creatures like Lactobax et al., C-section kiddos' first experience with bacteria tend to be more on the skin microbiota side (think Stapholococcus, etc). Of course, young humans' tendency to put everything in their mouth levels out the playing field eventually-- or does it?

"That looks like it needs to be licked. Please, allow me."

It depends. Genetics plays a big role, but C-section babies may have an increased risk of allergies, autoimmunity and other inflammatory conditions later down the line. Of course, it's not a given that all C-section babies will have issues -- it's just a possible consequence of the all-at-once colonization they're subjected to at birth. Babies who made their way through the vaginal canal tend to have slower, steadier rates of colonization, which is a bit easier on a newly forming immune system. If you think about dogs, there are some you'd call "well-socialized," and others not so much. The ones who weren't introduced to other dogs when they were young can be more aggressive and unsure of how to act around their peers. The same goes for immune systems and bacteria: tolerance of others is an important attribute among the single-celled, as well.

It's also important to realize that as we get older, our microbiotas differentiate more. Newborns generally have the same bacteria EVERYWHERE-- so the microbiota of their genitals, skin, guts, mouths, and other orifices all look about the same in terms of microbial diversity. It's not until everything has the chance to mature that certain kinds of bacteria become more popular in different parts of the body.

So, that's the story. Once we leave our mamas' bellies, our bodies become hosts to all kinds of microbes, which form cliques in different parts of our bodies. In most cases, everybody lives happily ever after.

Hooray!