Some of the most intriguing work has been done on autism. For decades, doctors, parents, and researchers have noted that about three-quarters of people with autism also have some gastrointestinal abnormality, like digestive issues, food allergies, or gluten sensitivity. This recognition led scientists to examine potential connections between gut microbes and autism; several recent studies have found that autistic people’s microbiome differs significantly from control groups. The California Institute of Technology microbiologist Sarkis Mazmanian has focused on a common species called
Bacteroides fragilis, which is seen in smaller quantities in some children with autism. In a
paper published two years ago in the journal
Cell, Mazmanian and several colleagues fed
B. fragilis from humans to mice with symptoms similar to autism. The treatment altered the makeup of the animals’ microbiome, and more importantly, improved their behavior: They became less anxious, communicated more with other mice, and showed less repetitive behavior.
Exactly how the microbes interact with the illness—whether as a trigger or as a shield—remains mostly a mystery. But Mazmanian and his colleagues have identified one possible link: a chemical called 4-ethylphenylsulphate, or 4EPS, which seems to be produced by gut bacteria. They’ve found that mice with symptoms of autism have blood levels of 4EPS more than 40 times higher than other mice. The link between 4EPS levels and the brain isn’t clear, but when the animals were injected with the compound, they developed autism-like symptoms.
Mazmanian, who in 2012 was awarded a MacArthur grant for his microbiome work, sees this as a “potential breakthrough” in understanding how microbes contribute to autism and other neurodevelopmental disorders. He says the results so far suggest that adjusting gut bacteria could be a viable treatment for the disease, at least in some patients. “We may be able to reverse these ailments,” he says. “If you turn off the faucet that produces this compound, then the symptoms disappear. That’s what we see in the mouse model.”
Scientists have also gathered evidence that gut bacteria can influence anxiety and depression. Stephen Collins, a gastroenterology researcher at McMaster University in Hamilton, Ontario, has found that strains of two bacteria,
lactobacillus and
bifidobacterium, reduce anxiety-like behavior in mice (scientists don’t call it “anxiety” because you can’t ask a mouse how it’s feeling). Humans also carry strains of these bacteria in their guts. In one study, he and his colleague collected gut bacteria from a strain of mice prone to anxious behavior, and then transplanted these microbes into another strain inclined to be calm. The result: The tranquil animals appeared to become anxious.
