Stress can send your stomach into a painful tailspin, causing cramps, spasms and grumbling. But trouble in the gut can also affect the brain.

This two-way relationship may be an unlikely key to solving one of medicine's most pressing—and perplexing—mysteries: autism. Nearly 60 years after the disorder was first identified, the number of cases has surged, and the United Nations estimates that up to 70 million people worldwide fall on the autism spectrum. Yet there is no known cause or cure.

But scientists have found promising clues in the gut. Research has revealed striking differences in the trillions of bacteria - collectively known as the microbiome—in the intestines of autistic and healthy children. But the gut bacteria in autistic individuals aren't