Scientists in California say they have transformed understanding of Parkinson’s disease.
Their animal experiments, published in the journal Cell, suggest the brain disorder may be caused by bacteria living in the gut.
The findings could eventually lead to new ways of treating the disease, such as drugs to kill gut bugs or probiotics.
Experts said the results opened an “exciting new avenue of study”.
In Parkinson’s disease the brain is progressively damaged, leading to patients experiencing a tremor and difficulty moving.
Researchers used mice genetically programmed to develop Parkinson’s as they produced very high levels of the protein alpha-synuclein, which is associated with damage in the brains of Parkinson’s patients.
But only those animals with bacteria in their stomachs developed symptoms. Sterile mice remained healthy.
Further tests showed transplanting bacteria from Parkinson’s patients to mice led to more symptoms than bacteria taken from healthy people.
Dr Timothy Sampson, one of the researchers at the California Institute of Technology, said: “This was the ‘eureka’ moment, the mice were genetically identical, the only difference was the presence or absence of gut microbiota.
“Now we were quite confident that gut bacteria regulate, and are even required for, the symptoms of Parkinson’s disease.”