Autism is primarily a disorder of the brain, but research suggests that as many as nine out of 10 individuals with the condition also suffer from gastrointestinal problems such as inflammatory bowel disease and “leaky gut.” The latter condition occurs when the intestines become excessively permeable and leak their contents into the bloodstream. Scientists have long wondered whether the composition of bacteria in the intestines, known as the gut microbiome, might be abnormal in people with autism and drive some of these symptoms. Now a spate of new studies supports this notion and suggests that restoring proper microbial balance could alleviate some of the disorder's behavioral symptoms.

At the annual meeting of the American Society for Microbiology held in May in Boston, researchers at Arizona State University reported the results of an experiment in which they measured the levels of various microbial by-products in the feces of children with autism and compared them with those found in healthy children. The levels of 50 of these substances, they found, significantly differed between the two groups. And in a 2013 study published in PLOS ONE, Italian researchers reported that, compared with healthy kids, those with autism had altered levels of several intestinal bacterial species, including fewer Bifidobacterium, a group known to promote good intestinal health.

One open question is whether these microbial differences drive the development of the condition or are instead a consequence of it. A study published in December 2013 in Cell supports the former idea. When researchers at the California Institute of Technology incited autismlike symptoms in mice using an established paradigm that involved infecting their mothers with a viruslike molecule during pregnancy, they found that after birth, the mice had altered gut bacteria compared with healthy mice. By treating the sick rodents with a health-promoting bacterium called Bacteroides fragilis, the researchers were able to attenuate some, but not all, of their behavioral symptoms. The treated mice had less anxious and stereotyped behaviors and became more vocally communicative.

Researchers do not yet know how exactly gut bacteria might influence behavior, but one hypothesis is that a leaky gut may allow substances to pass into the bloodstream that harm the brain. In the mouse study, the probiotic may have helped reshape the microbial ecosystem and made the intestines more robust, preventing the leakage of such substances, says co-author Elaine Y. Hsiao, a microbiologist at Caltech.

So could autism one day be treated with drugs designed to restore a healthy microbial balance? Perhaps, but autism is the result of a “complex interplay of genetic and environmental factors,” explains Manya Angley, an autism researcher at the University of South Australia, so the solution may not be that simple. Caltech biologist Sarkis K. Mazmanian, co-author of the mouse study, agrees. “Many more years of work will be needed before we are confident that gut bacteria impact autism and whether probiotics are a viable treatment,” he says.