Link Between Gut and Oral Bacteria and Cognitive Impairment in Parkinson’s Disease
A study indicates that changes in bacterial populations in the gut and mouth may be linked to cognitive issues in individuals with Parkinson’s disease. Researchers found that certain oral bacteria and their virulence factors—molecules that assist bacteria in colonizing the body, some of which can trigger strong inflammatory reactions—were notably more abundant in the intestines of Parkinson’s patients experiencing cognitive decline.
“Our findings imply that fostering a healthy microbiome through regular oral hygiene, a balanced diet, and possibly targeted probiotic treatments could enhance disease management in Parkinson’s,” remarked Saeed Shoaie, PhD, the lead of the study at King’s College London, as noted in a university announcement. This research, titled “Microbiome signatures of virulence in the oral-gut-brain axis influence Parkinson’s disease and cognitive decline pathophysiology,” was published in Gut Microbes.
Parkinson’s can lead to various nonmotor symptoms, such as gastrointestinal issues like constipation or nausea, occurring years before the onset of motor symptoms. Alterations in the gut may influence the development of Parkinson’s through a communication system known as the gut-brain axis.
While the oral health of Parkinson’s patients affects their disease progression, the role of oral bacteria has received little attention. Interestingly, researchers have found Porphyromonas gingivalis, a well-known oral bacterium, present in the brains of individuals with Alzheimer’s, and active gum disease has been associated with cognitive deterioration.
“The connections between human gut and oral bacterial communities and neurodegenerative diseases are growing stronger,” noted Shoaie. “Disturbances in the gut-brain axis might trigger inflammatory and immune responses that lead to neuronal damage.”
Investigating Gut and Oral Microbiomes
The team analyzed gut and mouth bacteria by collecting stool and saliva samples from Parkinson’s patients with mild cognitive decline and dementia, comparing them to a control group of healthy individuals.
Those Parkinson’s patients with dementia exhibited notably lower diversity in their gut microbiome compared to those with only mild cognitive impairment. Additionally, there was a reduction in both microbial diversity and the total number of species in the oral microbiome among dementia patients.
In examining specific bacteria, the study noted limited overlap in bacterial species across the three groups, indicating that many species were either enriched or reduced in patients with mild cognitive impairment or dementia. This suggests that “these microbial alterations likely correlate with cognitive issues,” the researchers mentioned.
The predictive models based on changes in microbial species were effective at distinguishing between Parkinson’s patients with different clinical characteristics, with factors like age and sex enhancing the prediction accuracy.
“We don’t quite know if these bacteria are responsible for cognitive decline, or if Parkinson’s-related bodily changes facilitate their growth,” said Frederick Clasen, PhD, the study’s first author. “Nonetheless, our findings imply that these bacteria might actively contribute to worsening symptoms.”
Oral Bacteria Migration to the Gut
The researchers explored a concept known as oral-gut translocation, where oral bacteria enter the gut, an environment they typically don’t inhabit. Their findings indicated that the guts of patients with dementia had a higher prevalence of oral bacteria. Moreover, those experiencing cognitive impairments had increased levels of specific virulence factors from oral bacteria in their guts—molecules that help these bacteria colonize.
These identified virulence factors serve various purposes, including stress response, immune modulation, adherence, biofilm formation, and invasion. Some are crucial for bacteria to infect the cells lining blood vessels in the brain, while others contribute to producing LPS, a molecule that incites inflammation.
“These toxins could serve as biological markers for identifying Parkinson’s patients at a higher risk of dementia,” Clasen stated. “Potentially, they might also become targets for treatments aimed at protecting the brain by altering the gut environment.”
The study observed a decline in several bacterial types in the mouths of dementia or cognitive impairment patients compared to healthier individuals, which might indicate a deterioration in the healthy bacterial balance, making room for harmful bacteria.
“Our research provides strong evidence that the relationship between oral and gut microbiomes significantly impacts Parkinson’s and cognitive decline,” the team noted. “The transfer of oral microbes to the gut, along with their associated virulence factors, opens new paths for understanding disease mechanisms, possible biomarkers, and therapeutic options.”
Shoaie emphasized that “emerging evidence suggests the importance of maintaining oral and gut health to slow or mitigate neurodegenerative processes. As individuals with Parkinson’s increasingly rely on caregivers, routine practices like oral hygiene and proper nutrition may sadly be overlooked.”
