Your body is teeming with bacteria. Recent studies estimate that 39 trillion bacterial cells live in and on the human body, with over 1,000 species living in your gut alone. To put that in perspective, the average human body has only about 30 trillion human cells.
But don’t worry — having all that bacteria is a good thing! The bacteria inside of you, also known as your microbiota, are essential to your health. Disruptions to the microbiota are associated with obesity, allergies, liver disease, inflammatory bowel diseases and autoimmune diseases. Recently, researchers have come to understand that the bacteria inside of you are not only physiologically important, but also play a crucial role in the way you think, feel and behave.
In 2004, Japanese researchers at Kyushu University and Tokai University in Japan decided to look at the effect that gut microbes had on stress. To do this, they raised what are known as germ-free mice — mice grown in a completely sterile environment with no bacteria in their GI system. When the mice were restrained — a stressful event for a mouse — they had significantly higher levels of stress hormones compared to normal mice. Somehow, the bacteria in normal mice were reducing the amount of stress they felt. The researchers took it one step further by introducing bacteria into the germ-free mice and were able to show that the stress response went back down.
This study sparked a huge interest in the scientific community. Since 2004, dozens of researchers have conducted similar studies in animals. Their findings have consistently linked the microbiota with a wide range of cognitive effects. They have found decreased cognition, memory and social interactions in germ-free mice — and all these effects were reversed when bacteria were re-introduced. One study showed that germ-free mice even had a preference for fatty foods and ate higher amounts of calories from fat. Others have shown that an altered microbiota affects anxiety, depression and the response to pain.
Though the amount of research done on the human microbiota-brain connection is limited, there have been some initial promising studies that suggest changing your microbiota can have cognitive effects. A randomized, placebo-controlled study demonstrated that taking a probiotic, or supplement containing bacteria, decreased psychological distress and anxiety. When other researchers looked at fMRI scans of the brains of people taking probiotics, they saw a change in how their brains processed negative emotional stimuli. Patients with cognitive impairment from a disease known as hepatic encephalopathy showed cognitive improvement when their microbiotas were altered with antibiotics that selectively affected the gut. There is currently active research investigating the potential role that bacteria play in certain diseases such as Parkinson’s disease, chronic pain, mood disorders and even autism.
Scientists aren’t sure how bacteria are influencing our moods but there are some theories. One is that these bacteria are influencing our immune systems, which in turn release chemicals called cytokines that are known to affect the brain. Another theory is that the bacteria are directly interacting with neuronal cells in the gut. Our intestines are home to something known as the enteric nervous system, a collection of about 100 million neurons with extensive connections to our brains. Chemicals released by bacteria could be directly travelling to the brain and affecting us that way. Certain bacteria are even able to make neurotransmitters such as GABA, norepinephrine, serotonin, dopamine and acetylcholine — molecules that are central to brain functioning.
Before you run out and buy probiotics, keep in mind that we still know very little about the microbiota-brain connection. We don’t even know which bacteria of the 1,000+ species in our gut are the most important for brain functioning. More research will have to be done before we can confidently alter our microbiotas for improving brain function. What is becoming increasingly clear, however, is that what we have long thought of as the things that make us most human — our thoughts and emotions — are influenced by some of the least human organisms on the planet.
Tasher Losenegger is a first-year MD candidate at Weill Cornell Medical College in New York City. He may be reached at firstname.lastname@example.org. –appears periodically this semester.