The study, led by the University of Helsinki Institute of Biotechnology in Finland, is published in the journal Movement Disorders.
It involved 72 patients with Parkinson's disease and an equal number of matched, healthy controls.
More and more studies are discovering the huge influence that our gut bacteria - which vastly outnumber the cells of our body - have on our health: when they get sick, we get sick.
Parkinson's disease is a progressive motor disorder that develops when the brain loses cells that produce dopamine - a chemical that controls reward and pleasure and also regulates movement and emotional responses.
Parkinson's symptoms include trembling, stiffness, slowness of movement and problems with balance and coordination.
The disease rarely strikes before the age of 50 and gradually gets worse - to the point where everyday life and self-care becomes very difficult.
According to the National Parkinson's Foundation, up to 60,000 new cases of Parkinson's are diagnosed each year in the US, adding to the 1 million Americans who currently live with the condition.
Some clues already exist about the links between Parkinson's and gut problems. For example, as the study authors say in their paper, "gastrointestinal dysfunction, in particular constipation, is an important non-motor symptom" in Parkinson's disease, and "often precedes the onset of motor symptoms by years."
They also mention that recent research shows gut bacteria interact with parts of the nervous system via various pathways, including the enteric nervous system - the so-called "brain in the gut" - and the vagal nerve.
Highlighting their findings, lead author of the new study, Dr. Filip Scheperjans, a neurologist in the Neurology Clinic of Helsinki University Hospital, says:
"Our most important observation was that patients with Parkinson's have much less bacteria from the Prevotellaceae family; unlike the control group, practically no one in the patient group had a large quantity of bacteria from this family."
The team did not find out what an absence of Prevotellaceae might mean in Parkinson's disease. But they have many questions. For example, does this family of bacteria protect against the disease? Or does the disease wipe them out?
"It's an interesting question which we are trying to answer," says Dr. Sheperjans.
The team also found that levels of another family of bacteria called Enterobacteriaceae appear to be linked to severity of Parkinson's symptoms. They observed patients who had more difficulty with balance and walking tended to have higher levels of these bacteria.
Dr. Sheperjans and his colleagues are already planning further research to explore the connection between Parkinson's disease and gut bacteria.
They have begun to re-examine the same group of patients to find out if the differences in gut bacteria are permanent or whether they change as the disease progresses. If they do change with disease progression, this could help doctors give more accurate prognoses.
"In addition," Dr. Sheperjans says, "we will have to see if these changes in the bacterial ecosystem are apparent before the onset of motor symptoms."
And, he adds, they also want to discover the underlying biological mechanism between gut bacteria and Parkinson's disease.
They hope eventually that their findings will lead to new tests for Parkinson's and perhaps even new treatments to stop, slow or even prevent the disease by focusing on gut bacteria.
Funds from the Michael J. Fox Foundation for Parkinson's Research and the Finnish Parkinson Foundation helped finance the study.
In November 2014, Medical News Today learned of a stem cell treatment breakthrough for Parkinson's disease. A study involving laboratory rats suggests it may be possible to replace dopamine cells lost to Parkinson's disease by making them from embryonic stem cells and then transplanting them into the brain.
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