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My Microbiome Made Me Do It

Her microbiome made her do it.

Her microbiome made her do it.

My sister craves chocolate. Is it her or her microbiome?

Your microbiome – the 100 trillion microscopic organs living in your body – may weigh up to three pounds (ca. 1,400 grams). In other words, it may be about as big as your brain. It may affect a wide range of diseases including obesity, muscular dystrophy, diabetes, multiple sclerosis, and chocolate abuse

Your microbiome affects disease, but can it affect your behavior? Can it force my sister to eat chocolate? Let’s start with the case of the cat and the mouse.

Toxoplasma gondii is a single-celled parasite that cycles between cat and mice. To complete its life cycle, it needs to pass from cat to mouse and back to a cat again. Toxoplasma lives in the cat’s gut so it’s fairly easy to get from the cat to the mouse – it travels in cat poop.

But how does it get back to the cat? Mice are, of course, deathly afraid of cats and scrupulously avoid them. But not if they’ve been infected with toxoplasma. Then they start wandering around in the open and may even be attracted to the smell of cats. They’re much more likely to be eaten by a cat. Joanne Webster, who studies these things at Imperial College, calls it “fatal feline attraction”.

How does it work? It’s complicated. Somehow the toxoplasma generates dopamine that interferes with the mouse’s brain messaging system. The interference changes the mouse’s behavior.

Remember dopamine? It’s a neurotransmitter that (among many other things) works with the reward system in the brain. Dopamine helps reward us – mice and humans — for good behavior. When we do something well, our systems release dopamine and we feel a sense of pleasure and reward. Problems with the dopamine system can affect a range of behaviors, including ADHD and schizophrenia.

So, is toxoplasma related to schizophrenia? Jaroslav Flegr thinks so. Flegr found that schizophrenics were three to four times more likely to be inflected with toxoplasma than non-schizophrenics. Similarly, E, Fuller Torrey found that women infected with the parasite “were more likely to give birth to schizophrenics to-be.”

Flegr also studied toxoplasma and road accidents. “Both drivers and pedestrians who had been in accidents were almost three times more likely to be infected than comparable individuals who had not been.” They also had poorer reaction times and shorter attention spans. All of this could induce riskier behavior – just like in mice.

What does all this have to do with my sister? Toxoplasma is just one example of the microbes living in the human body. As Carl Zimmer points out, it’s possible that other microbes are influencing our behavior in myriad ways, including food cravings. As Zimmer puts it, “Maybe the microbiome is our puppet master.”

Did you ever wonder why humans are called “social animals”? Perhaps it’s because we can be more successful by collaborating and building societies. Or perhaps it’s because our microbes want to travel from one human to another. It’s much easier on our microbes if we’re clustered together.

And food cravings? Zimmer points out that different microbes like different kinds of food. Perhaps their desires drive our cravings. Zimmer writes that, “Many people crave chocolate fiercely, but it isn’t an essential nutrient. … Perhaps … certain kinds of microbes that thrive on chocolate are coaxing us to feed them.”

If true, does this mean that my sister is not responsible for her chocolate craving? Is she just an innocent bystander, manipulated by her microbial puppet master? It’s an intriguing question that I’ll save for a future article. In the meantime, if you get in trouble with the law, I’d suggest that you say, “My microbiome made me do it.”

(For a related article on zombie spiders, click here).

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