Doctors have used gut microbes from fecal matter transplants to treat bacterial superbugs like C. diff. Now, a new study suggests they could also help improve symptoms of autism spectrum disorder.

The research appears in the journal Scientific Reports.

Growing evidence that gastrointestinal issues may affect, or co-occur with, neurological conditions, including autism spectrum disorder (ASD), Parkinson's disease and Alzheimer's disease, has fostered hopes of treating the brain via the gut.

After 18 children received a 10-week treatment of fecal-sourced microbes, their GI symptoms diminished 80 percent.

Meanwhile, little by little, their core ASD symptoms improved.

"What we think is, by removing the chronic pain and discomfort — by removing the bad bacteria that are probably producing a variety of toxins — now, the children's brains are healthier, and now they can learn better," said co-author James Adams of Arizona State University.

Most notably, those improvements were still evident when the team retested the children two years later.

"Some had a 45 percent improvement; some were 60, 70 percent improvement; some were only 20 or 30 percent; but, on average, a 45 percent reduction. And that's huge," Adams said.

Adams called the treatment novel because it addresses core ASD symptoms and provides substantial improvement long-term.

Currently, children with ASD receive a combination of behavioral, speech and social therapy, along with dietary, nutritional and medical treatments. But no medication treats ASD's core symptoms: language problems, social disconnection and repetitive behaviors.

The gut microbiome provides one route by which environment plays a role in ASD.

Prior research has shown that children with ASD have distinctive gut microbiomes compared to neurotypical children, and mouse studies have found gut microbes and their associated chemical compounds can affect behavior.

The researchers conceded the small, open study was potentially vulnerable to placebo effects and noted that 12 of 18 participants had adjusted their medication, diet or nutritional supplements in the two-year interim. However, they found these effects to be minor compared to the strength of the observed outcomes.

They are now developing a larger, double-blind study involving adult subjects.