We have known for centuries that we share our bodies with microbes. But we always assumed they lived in places, like skin, mouth, and gastrointestinal tract, which are “outside” our bodies, and that they just use us for housing. Living on us but not interacting with us.

However, new findings from the Human Microbiome Project and other lines of study have strongly suggested that our relationships with our microbes are much closer than we thought. Much closer.

Who, or what, are we?

DNA studies of human tissue have revealed that, at least in number of cells, there are nearly as many of  “them” as “us”. But they are too small for us to see them. But, for instance, about 90% of the cells in the gut are microbes!

Not only that, but microbes seem to inhabit every tissue of our bodies, albeit in small numbers (at least compared to skin and guts). And yes. This includes the brain.

What are they doing there?

 

Most of our microbes are in our guts.

 

There are hundreds of different species of microbes that include bacteria, viruses, and fungi, which inhabit our gut. Because they are so numerous (and because they can be studied in things like poo), we know more about the populations of microbes in our guts than in the other parts of our bodies. The vast, vast majority of them are peaceful, and some of them make nutrients (short chain fatty acids) and vitamins (such as vitamin K) that we need. Some make our food easier to harvest energy from (great in a famine, but not if we are trying to lose weight). Others make neurotransmitters and hormones, such as serotonin, histamine, and GABA, which can influence immune cells and other cells in our guts. This may be one way microbes affect our behavior.

 

Microbes affect our behavior?

The first clue that microbes can influence behavior came from a study showing that that Toxoplasma gondii infections in mice make them less afraid of cats. Why? It turns out Toxoplasma lives in cats, and part of its life cycle involves being excreted in cat poo, which is then eaten by mice. When a cat eats the mouse, the toxoplasma gets back into its favorite host. So if mice are less afraid of cats, the cats have a better chance of eating them. Which is great for the Toxoplasma, if not for the mouse.

So far there are no examples of microbes being that specific about the way they change behavior in humans. However, in rodents and people, having an imbalance in the microbial species in the gut (called “dysbiosis”) can lead to anxiety. This effect is mediated by a nerve that connects the gut and brain (the vagus nerve), and it is thought that the anxiety is important as a signal that something is not right in the gut. Dysbiosis in humans is pretty common, thanks to antibiotic use, food additives and pesticides, and generally poor nutrition.

This means that what is going on with our microbes in our guts has very important implications for mental health, especially anxiety. It’s striking how many people suffer from stomach and gut disorders, dysbiosis, and anxiety symptoms. Many more people suffer anxiety without really knowing why, and are feeling frustrated and helpless because therapies directed towards the brain aren’t helping. It might be a good idea to start thinking about what is going on in their guts!

Microbes seem to play roles in disease, but not only the way we used to think.

When we think of microbes and disease, we think of things like the flu, food poisoning, and other infections. But the vast majority of microbes in our body are actually helping prevent infections with dangerous microbes. Each part of our body (nose, gut, skin, vagina) has it’s own specific population of microbes, that are unique to each of us. Those microbes know who is supposed to be there, and who isn’t. A healthy balanced population of microbes seems to act like police, by preventing dangerous bacteria from overgrowing and causing disease. In this way, they are able to keep us healthy in the sea of micro-organisms that we live in.

Can we influence the microbes in our guts?

Bacteria have receptors for our hormones and neurotransmitters. This means that what is going on in our bodies can influence their growth and possibly behavior. For instance, the neurotransmitter norepinephrine, which is released when we are stressed, can make certain pathogenic bacteria grow faster and be more virulent (cite Mark).

Recent studies indicate that what we eat influences the types of species, and how much they grow. So this is one more important reason to eat a healthy diet. Different kinds of foods support different species of bacteria, helping to promote “diversity” (which is good).

On the other hand, antibiotic and pesticide residues in food can be toxic to microbes, and this is one of the most compelling reasons to eat organic-type foods. Similarly, it’s important to avoid antibiotics (including “sanitizers”) unless they are truly necessary. Antibiotic overuse is a big contributor to the development of dysbiosis. I will write more on this in a future blog, because dysbiosis is turning out to be a major player in chronic disease, from gut inflammation to diabetes, to neurodegenerative diseases.

The bottom line for staying healthy, then, is to work with our microbes by managing stress and eating a healthy varied diet.