What Is Your Gut Telling You? Exploring the Role of the Microbiome in Gut–Brain Signaling

On 6 June 1822, French Canadian fur trade voyageur Alexis St. Martin was shot accidentally in the stomach at an American Fur Company store on Michigan’s Mackinac Island. The blast left a gaping wound in St. Martin’s abdomen. St. Martin eventually recovered from the gruesome accident, but the wound never closed completely, leaving a small permanent opening in his stomach wall. His surgeon, William Beaumont, began monitoring gastric secretions through this opening in St. Martin’s body. Beaumont, who would later become known as the father of gastric physiology, would attach various types of food to a string and suspend them through the hole. Later he would pull out the string to see what portion of the food had been digested. During these experiments, Beaumont noticed that St. Martin’s mood seemed to affect how quickly he digested food. When St. Martin was irritable, for instance, food broke down more slowly.2 These early observations provided the first clues of crosstalk between the brain and the gut. Researchers later called this bidirectional communication system the gut–brain axis. Over the years, studies have revealed that the brain influences the gastrointestinal (GI) tract through several mechanisms that involve the nervous and immune systems. Only recently have scientists recognized the importance of a third component to the gut–brain axis: the trillions of bacteria, viruses, archaea, and eukaryotes that make up the gut microbiome. In little more than a decade, researchers have uncovered intriguing associations between gut bacteria and a host of neurological disorders and psychiatric conditions. These include depression, anxiety, autism spectrum disorders (ASDs), and Parkinson’s disease. Most of the early research on the microbiome–gut–brain axis has been conducted in rodents. Germ-free mice—which are born in sterile conditions and free of all microorganisms—are popular for gut flora research because scientists can inoculate the mice with specific microbes and watch what happens. Now, additional researchers are beginning to probe the connection in humans. Outside neuroscience, gut microbiome research in laboratory animals and humans is changing the way some environmental health scientists view the effects of environmental exposures on neurodevelopment and brain chemistry.