Friday night, work finally done, I sit down in my favourite pub and order a pint. I’m tired, maybe a bit grumpy. A couple of pints in and I’m happy, my energy levels are up, I’m chatting to everyone vibrantly. A couple more pints and I’m dragging myself home somewhat stupefied.

Alcohol is just one of the many substances that can affect our behaviour in our daily lives, along with coffee, sugar, and any kind of medication. This is nothing new to us. But what about other living organisms? Can they have the same impact on us?

It’s something that’s often been drawn on in popular science fiction: if you’ve ever watched The Walking Dead (with the lovely Andrew Lincoln), 28 Days Later (great cult film) or The Happening you’ll have come across the idea. But could this ever happen in real life?

In fact, It would appear that there are many examples of this in the invertebrate kingdom: Cordyceps is a fungus that makes its ant host climb to the top of a plant, where it then bursts out of the ant’s head, raining spores down onto the floor below and thus infecting other unsuspecting ants. Parasitic wasps are another chilling example: they lay their eggs inside the tobacco hornworm; the hornworm then not only changes its feeding behaviour to suit the wasp parasite, but also begins to act as a bodyguard to the wasp larvae that emerge from its skin, shielding and protecting them from potential threats.

A tomato hornworm covered in parasitic wasp eggs
Creative Commons

These behavioural changes are impressive, but invertebrates do have pretty simple neural systems. What about something with a more complex brain, like a mammal? Well Seoul virus has been shown to increase aggressive behaviour in male rats, making them more likely to fight, possibly because the virus transmits via blood and saliva. And several parasites, both single-cell and larger worms, have been shown to reduce fear and anxiety behaviours in rats and mice.

So if microbes can influence behaviour in small mammals, surely this could apply to larger mammals, like us humans? Anyone who has read To Kill a Mockingbird in secondary school will remember the unnerving scene in which Atticus Finch bravely shoots a rabid dog. This scene is particularly harrowing because of the effects that rabies, a viral infection, can have on the nervous system of humans. People infected with rabies experience severe agitation and phobic spasms, often followed by death.

History is filled with tales of neurosyphilis, the third stage of syphilis, a sexually-transmitted bacterial infection. Known as ‘The Great Pox’, this disease was widespread before the discovery of antibiotics, and would cause irritability, deterioration of cognitive function and concentration, and dementia.

There are many ways by which microbes can influence us – we’re all a bag of microbes, and infection is a part of normal human variation.

And if neurosyphilis and rabies seem far removed from us: Toxoplasma gondii is a parasite that currently chronically infects around one-third of people in London – that means that you, or several people you know, are likely to be unknowingly infected. T. gondii has been suggested to cause subtle personality changes in men and women; in people who are at risk of schizophrenia and attempted suicide, there is some evidence that infection may increase the risk or worsen the mental illness.

You may wonder how these little microbes can impact something as uniquely human as our behaviour, but there are many ways by which microbes can influence us – we’re all a bag of microbes, and infection is a part of normal human variation. Gut microbes might influence brain chemicals via the gut-brain axis. Microbes that infect babies just before or after birth may cause small lesions in the developing brain. Microbes may go to the part of the brain involved in emotion and directly affect brain chemicals, or indirectly via the immune response. So maybe the brain-snatching microbes loved by science fiction aren’t so fictional after all.