The exact cause of Parkinson’s disease isn’t known.

Genetics and environment are possible factors, but now researchers say gut bacteria could contribute to the nervous system disorder.

Researchers at the California Institute of Technology (Caltech) published a report today in the journal Cell detailing their discovery of a link between intestinal bacteria and Parkinson’s disease (PD).

Changes in bacteria, or the bacteria themselves, contribute to — and may even cause — motor skill decline, the scientists concluded.

Up to 1 million Americans are affected by PD. It is the second most common neurodegenerative disease in the United States.

Symptoms include difficulty walking, tremors, and speech changes.

People with PD have a build-up of alpha-synuclein (αSyn) protein within cells in the brain and gut, and cytokines (inflammatory molecules) within the brain.

About 75 percent of people with PD experience gastrointestinal (GI) abnormalities such as constipation before those symptoms appear.

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A gut check

Sarkis Mazmanian, Ph.D., a Caltech microbiologist, and Heritage Medical Research Institute investigator, published the report in Cell.

He said the gut is home to “a diverse community of beneficial and sometimes harmful bacteria” — the microbiome — that is vital for immune and nervous system function.

“Remarkably, 70 percent of all neurons in the peripheral nervous system — that is, not the brain or spinal cord — are in the intestines, and the gut's nervous system is directly connected to the central nervous system through the vagus nerve,” Mazmanian said in a statement.

GI problems typically precede motor control symptoms, so his team wanted to explore the potential role of gut bacteria.

The study, done on mice, found that rodents without a microbiome (germ-free mice) had normal motor skills even when they had αSyn protein buildup.

Part of the study, though, involved working with fecal samples from humans with Parkinson’s. When the human microbiome samples were put into the germ-free mice, they began to show Parkinson’s symptoms.

“The fact that you can transplant the microbiome from humans to mice and transfer symptoms suggests that bacteria are a major contributor to disease,” Mazmanian added.

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Gut health already part of treatments

Dr. Marie Saint-Hilaire, a neurology professor at Boston University Medical Campus, said other studies suggest PD can start in the gut.

Saint-Hilaire is a member of the American Parkinson Disease Association Scientific Advisory Board.

“Constipation is one of the nonmotor signs of PD which can start years before the motor signs,” she told Healthline. “We do use probiotics for treatment of constipation in PD. It [constipation] can be distressing to patients and is one of the nonmotor symptoms that gets discussed commonly during visits.”

Saint-Hilaire referenced a 2015 study that concluded people who had their vagus nerve severed to treat gastric ulcers had a lower risk of Parkinson’s than those who had a partial transection.

She said animal findings must be replicated in humans to confirm the link, so more studies would need to be done.

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Better insight into genetic factors

In related news, researchers at the Ann Romney Center for Neurologic Diseases at Brigham and Women’s Hospital, found that glucocerebrosidase (GBA) gene mutations that are a known Parkinson’s risk factor have a powerful influence over cognitive decline development.

The study was published last month in Annals of Neurology.

The study’s authors said the research may help to better target pharmaceutical trials more effectively.