Are we masters of our own fate… or have our brains been zombified by microbes?
To learn more about Cordyceps and similar mind-controlling parasites, watch Zombie parasites on YouTube.
Dethlefson and colleagues wrote that “from the microbial perspective…the distinction between human health and disease is important only as far as it affects microbial fitness.
This raises the possibility that microbes manipulate us for their own benefit. Examples abound of parasites like Cordyceps exploiting their insect hosts. But do microbial parasites affect us humans in the same way?
Dethlefson and colleagues write that microbiota are “prevented (usually) from exploiting the host to obtain additional resources,” citing the immune system as “the most conspicuous set of anti-exploitation adaptations involved in human–microbial symbiosis.”
The fact that we need immune “anti-exploitation” adaptations is a clue that we are engaged in a never-ending arms with harmful microorganisms.
But the immune system only gets us so far. For example, infection by the protozoan Toxoplasmosis provides an excellent example of a parasite that manipulates the brain and behavior of mammals that it infects. Infected rabbits for instance lose their innate fear of bobcat urine. Rabbits that are attracted to feline predators have a distinct disadvantage when it comes to fitness. Not so for the parasite. It’s fitness is increased as it completes its life cycle. Our closest relative, the chimpanzee, also seeks out the urine of their main predators, leopards, when infected by Toxo. In the United States, almost a quarter of human adults are infected with Toxoplasmosis. Problem? Perhaps.
How about our microbiomes, the complex microbial communities that mainly inhabit our guts? Are they exploiting our brains and behaviors? Indeed, microbes are suspected to be involved in depression and anxiety, demonstrated experimentally in mice. Is this manipulation?
Bruce-Keller (2015) and colleagues showed that the microbiome is responsible for marble burying behavior in mice that were fed a high fat diet (HFD). Anxious mice like to bury marbles. The implication of this study is that microbes can make mice anxious and depressed.
Does this mean that microbes cause depression in people? Drawing on these kinds of studies, investigators have given probiotic microbes to people with depression. They seemed to get better!
Akkesheh and colleagues gave Lactobacillus acidophilus and Lactobacillus casei and Bifdobacteria to depressed patients in Iran. Probiotics apparently made patients less depressed. (The dark blue bar indicates placebo, and the light blue bar indicates the probiotic group. They used the Beckwith Depression Inventory to measure depression.)
How might toxoplasmosis affect chimp brains? How does Lactobacillus brighten mood? We don’t know, but there are clues.
Bacteria make neurotransmitters – the same ones that we use:
Presumably bacteria evolved the signaling molecules like GABA, norepinephrine, and serotonin first. We eukaryotes might have evolved nervous systems using the same molecules because they conveyed useful information about the doings of microbes (but who knows.)
My research collaborators proposed that gut microbes might affect what we decide to eat.
“Joanne Webster and colleagues from Imperial College, UK, and the University of Leeds, UK, make the case that Toxoplasma may be a contributory factor in some cases of human schizophrenia given its presence in the brains of infected individuals and our long lifespan”
” Like microscopic puppetmasters, microbes may control the eating behavior of hosts through a number of potential mechanisms, including reward pathways, production of toxins that affect mood, and hijacking of neurotransmitters via the vagus nerve”
3) Melancholic Microbes: a link between gut microbiota and depression?
“Whether fecal transplantation of microbiota from animals or other models of depression could also alter behavior of the recipient is an intriguing possibility”
“A significant OR of 1.91 (95% CI 1.49–2.44, P < 0.00001) was found, indicating an increased prevalence of T. gondii infection in heroin addiction”
Writing for next Tuesday, September 13th (pick one):
- Anxiety and depression often go together. Some people think that anxiety and depression might be adaptive (beneficial) for the patient. Do you? What do you think about this hypothesis: anxiety and depression might be adaptive for microbes and harmful for humans?
- Suppose you want to test the hypothesis that food cravings are caused by manipulative gut microbiota? What is the best way to do this?