The human body is made up of roughly 10 trillion cells. That's a huge number, but it's dwarfed by the 40 trillion or so bacterial cells that live on and inside the body.
In recent years, scientists have learned that our bacteria — known collectively as the microbiome — are intimately involved with many aspects of our health, including the robustness of our immune system and our risk of developing asthma and allergies. Now, new research suggests that the microbiome may also play a key role in neurological and psychological disorders.
"At a basic neuroscience level, all the key mechanisms of the brain have been shown to be regulated by the microbiome," says John Cryan, a neuroscientist at University College Cork in Ireland.
As scientists investigate how the microbiome impacts our bodies and minds, some experts envision fundamental changes in the way doctors diagnose and treat disease.
"Someday soon, you and I will go into the doctor's office, and the doctor will prescribe medicine that won't be a chemical from a lab, but may be live bacteria," says Sarkis Mazmanian, a microbiologist at Caltech in Pasadena, California. "In addition to blood work, yearly physicals may include fecal samples to profile the microbiome," tracking changes in bacteria to predict your risk for illness, sometimes long before symptoms appear.
The positive health effects of live bacteria have already been hinted at through research on prebiotics and probiotics, which scientists think may play an integral role in your immune system and on how you feel. New studies show that far more diseases may be impacted by the microbiome than previously understood.
"Bacteria could eventually be used in lieu of chemical treatments," Cryan says. He's even coined a term to describe the possibility: psychobiotics, adding that, "Within five years, we'll have specific microbial-based treatments."
Breakthroughs in Bacteria
In December, Mazmanian's lab revealed preliminary evidence linking the microbiome to Parkinson's disease, which as a neurodegenerative disorder has long been believed to be a disease of the brain, not the gut.
"At first we were in disbelief with the results," Mazmanian says of the research, which was done in rodents. "We kept testing and it kept leading us back to the same conclusion."
More investigation is necessary, but this new understanding of Parkinson's could help researchers find better ways to treat it. Future studies could identify which microbes are involved, and then researchers might be able to develop targeted bacterial treatments to help combat the disease.
This result isn't just true of neurodegenerative disorders: There's also evidence that disturbances of the microbiome might be a factor in psychological illnesses, too. Cryan has shown that a particular bacterium works as well as a popular antidepressant in reducing levels of stress hormones in animals subjected to stress tests.
Furthermore, researchers at Baylor College of Medicine found in a 2016 study that one species of gut bacteria causes social deficits in mice similar to autism in humans. By adding the bacteria back to the guts of affected mice, researchers were able to reverse their behavioral deficits.
Although further evidence linking the microbiome to these and other illnesses in human trials will be necessary, we're already seeing changes in the way doctors treat certain conditions.
Take infections with the deadly bacterium Clostridium difficile, which each year kill 14,000 Americans. The condition is notoriously difficult to treat, but doctors are finding it can often be cured with fecal transplants — fecal material taken from someone with a healthy microbiome and delivered to the patient's colon. By introducing helpful bacteria, the treatment can combat the illness better than antibiotics can.
Out of Balance
Martin Blaser, director of the Human Microbiome Project at New York University, points to several causes for the imbalance of someone's microbiome, including the overuse of antibiotics, the use of antibacterial rinses for our food and sanitizers for our skin, and even the popularity of cesarean sections. (Ordinarily, we get our microbiomes from our mothers as we move through the bacteria-laden birth canal during childbirth — but, of course, that doesn't happen to babies born via C-section.)
As a result of these factors, some species of bacteria are disappearing in developed countries, leading to a disturbance of bacterial populations in the body. Helicobacter pylori, for example, is one such bacterium that seems to disappear with the use of antibiotics. Although it's associated with stomach ulcers, esophageal diseases and even cancer seem to spike when it's absent from the body.
Blaser explains this is not uncommon. He believes the loss of diversity in our microbiomes is a factor in a range of modern plagues that includes diabetes, cancer, and even obesity.
"Microbial bacteria were the first form of life on earth, and the most numerous," Blaser says, and everything that subsequently evolved, like humans, had to take bacteria into account. "The whole biosphere requires the presence of bacteria." We're still just learning what that might mean.
Lois Parshley is a science journalist. Follow her on Twitter @loisparshley.
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