All the little beasties



Recently, a friend remarked to me about the microbes in our guts. She mentioned that the small intestine has no bacteria, that only the large intestine does. (that’s wrong.) I think that’s a misconception she has because of this idea ‘out there’ of gut overgrowth—the idea that too many bacteria (or yeast) flourishing in the small intestine is an unhealthy sign.

I don’t know much about overgrowth in the small intestine. But I for sure know about the human microbiome. It’s a fascinating topic and the numbers of bacteria and archaea in and on us are staggering. The mass is staggering, and their roles in our bodies (and in the world) are too.

You’ve probably heard that 90% of the cells in our body (in the space that our body occupies) are microorganisms, and only 10% are human cells. OK. This is disgusting. But take a breath, because microorganisms are tiny compared to human cells. A human egg is the size of the period at the end of this sentence, and although a human egg is a very large cell (about four million cubic microns), even average-sized human cells are still freaking big, like four thousand cubic microns.

Bacteria, on the other hand, clock in at one single cubic micron.

So, OK, there could be ten times as many bacteria (and archaea… and fungi and protists and small mites and worms and other lovelies in and on us), but each of our cells is several orders of magnitude bigger than any prokaryotic cell. So, in the end, only about 1% of our mass is not what we think of as ‘us.’ A few pounds.

(Tangent… But what if–wait, what if cells, are us? Oooh.  If we are no more than cells working together, why would a human cell contribute to consciousness, awareness, ‘being human,’ any more than a prokaryotic cell? Whoa. You know what, there’s research about this too… but…  too big a topic. Maybe later.)

OK. Guts and bacteria. So, there’s the small intestine and the large intestine, and these are divided into segments (like the duodenum right past the stomach, and the jejunum further down). There’s a duct near the stomach (the bile duct) and the appendix too.

And the whole intestinal tract is over twenty-five feet long. Here’s WebMD’s pages on the intestines.

So depending on where you look in the glorious mess of guts inside the human body, the distribution and numbers of microbes changes. Near the food (incoming!) or the bile, or where there’s no oxygen, or in the colon where digestion is over, the make-up of microbes changes. The numbers change too. Here’s an article with the numbers.

We’re covered in bacteria, inside and outside. What’s in our mouths is hugely diverse, hundreds of species, and different from our neighbors’ mouths. The stomach and duodenum are diverse places too—they get first crack at food, and that matters to the microbial community! There’s lots of different Proteobacteria in these niches. By the time digested food reaches the large intestine the community is down to the diehard phyla that thrive in anoxic environments—Bacterioidetes and Firmicutes in this case.

There’re different populations on our skin, in our underarms, our genitals, our belly buttons. The secretions of our eyes and ears shifts the microbes there. Our sinuses are moist and salty places that Staphylococcus loves. Our nails can feed fungi (a eukaryote). Toe jam. Need I say more?

If you removed every human cell from the space that body occupies, you would still see an outline of every surface of that body, an outline from the bacteria and archaea.

And they outnumber us ten to one!

And that’s why microbial ecology is so fascinating. 🙂

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