Biology for Writers: The Human Microbiome

Good morning, everyone! Happy Monday! It’s the second week of November (already!), so here I am with a quick post on a bioscience topic which may be of interest to some of the writers who read my blog (and don’t have that much background in science). Let’s get right to it!

The Overview

One of the biggest topics in science right now is the human microbiome. What is a microbiome, you ask? Well, it’s simply the entire set of microbes that inhabit the human body.

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Yes, that’s right–there are microbes in you, right now, and they’re even supposed to be there! As a matter of fact, there are more bacteria, archaea (basically extreme-environment microbes), and unicellular fungi in your body than there are human cells. Yes, you heard that right–we humans are more microbial than human. Blows your mind a little, doesn’t it?

So what do these microbes do for us? Are they good for anything, or do they just sit there? Well, my friends, I’m glad you asked. Let’s take a look at some of the parts of our bodies where our normal microbes are most influential.

The Mouth

We all know about dental plaque, and brushing our teeth so we don’t get it. (Seriously, I hope you all brush your teeth, ’cause plaque is disgusting.) But your mouth also contains 50-100 billion normal bacteria of at least 500 different species, mostly anaerobes, or bacteria that don’t require oxygen to live. (In fact, some anaerobes die when exposed to oxygen.) Streptococcus mutans and S. sanguinis, the organisms that cause plaque when they build up into a biofilm, are both “facultative anaerobes,” which means that they can either use oxygen or not depending on conditions in their environment (the mouth). Many other normal mouth bacteria can cause problems for the human host if there is bleeding in the mouth, or some other abnormal condition, bringing up an important point for the microbiome in general: Microbes that are normal inhabitants of our bodies (“normal flora” or “microbiota”) can be pathogenic if they are moved to a different spot or if something abnormal happens. These are called “opportunistic pathogens,” and we’ll see more of them as we move through the body.

The Gastrointestinal Tract

This is one of the places where the human microbiome is most important and best characterized; a lot of work has been done linking the gut microbiome composition to everything from diet to Parkinson’s disease. And gut microbes do a lot for us. For instance, we can’t digest vegetables without our gut microbes, and other microbes make vitamin K for us. That’s pretty darn helpful of them, don’t you think?

But when I say “gut microbiome,” what am I talking about? It turns out that not every part of the human body is colonized by microbes; accessory organs such as the liver and pancreas are sterile, and the stomach has a very low level of microbes due to the high acid content there (the stomach’s pH, a measure of its acidic content, is about 2–so very acidic). Not many things can survive the acidic environment of the stomach, which is why it’s great for digestion. One Helicobacter species, however, survives by burrowing into the stomach lining and secreting basic compounds, which neutralize the surrounding acid to create a neutral pH (good for life). This species can also cause stomach ulcers when it has lived in the stomach lining for a long time.

So if the stomach doesn’t have very many microbes, where is this gut microbiome I’ve been telling you about? Most of the gut microbiota live in the ileum (the last part of the small intestine) and in the colon (also known as the large intestine), where, as I mentioned, they help us digest our food, give us nutrients, and take up space so invaders can’t enter. This is known as a “mutualism,” in which both symbiotic partners benefit from their relationship. How do the bacteria benefit from us? Well, most parts of the body are at a neutral pH, which as I’ve already said is good for life, and they remain at a constant warm temperature, which allows microbes to grow.

I also just want to point out quickly that taking too many antibiotics can interrupt your gut microbes, and thus actually make you susceptible to sickness, since with space freed up by antibiotic treatment, pathogens can easily colonize your gut. Since this post is mainly about the microbiome, that’s all the space I can give to this extensive topic, but if you’re interested in learning more, feel free to comment below!

The Skin

The skin is one of our first defenses against microbes, since we are constantly sloughing off dead skin cells, but it is also colonized by many microbes. Similar to the gut microbiome, these take up space on your skin, preventing infection by pathogens. The skin microbiome also includes Staphylococcus aureus, an opportunistic pathogen which normally lives on the skin surface, but can cause problems when it penetrates deeper into the body. For instance, if you have a deep puncture wound, your normal S. aureus may get under your skin (literally) and cause nasty carbuncles and things when it infects you. So if you’re writing a novel and your character gets wounded, this is one thing that could follow that up if you want to give them extra torture be realistic about the consequences of wounding.

Find out More!

Here are some resources if you’d like to find out more about the human microbiome!

Human Microbiome Project–This would be a great resource if you want to find out some of the specific microbes that exist in each place on the human body (there are many, many more microbes than the ones I talked about today).

The Gut Microbiome in Health and Disease–A scientific paper about the gut microbiome. (Please note this is intended for a scientific audience, so it may be a bit dense.)

When Gut Bacteria Change Brain Function–An interesting general-audience article about how gut bacteria impact the brain.

I will continue to add more resources here as I find them! If you find interesting resources, feel free to let me know in the comments and I will consider adding them to this page.

That’s it for my first Biology for Writers post! Are you thinking of using the microbiome in your story? If so, how? Is there anything else you’d like to know about the microbiome? (I’m definitely not an expert, but I can point you to more resources if you’d like!) Are there other biology topics you’d like to see addressed in this monthly post series? Let me know in the comments!

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