Oral and gut bacteria linked to cognitive decline in Parkinson’s, study finds

Bacteria in the mouth and gut may contribute to worsening memory and thinking problems in people with Parkinson’s disease, researchers have found.

Specific bacterial changes appear to be linked to the progression from mild memory symptoms to dementia – a common and distressing feature of the condition – raising the possibility they could serve as early warning signs or targets for future treatment.

The research suggests that harmful bacteria from the mouth may migrate to the gut and release toxins that damage tissue, trigger inflammation and potentially affect the brain, worsening cognitive symptoms in Parkinson’s.

Dr Frederick Clasen is research associate at the Quantitative Systems Biology Lab at King’s College London, who led the research.

He said: “We don’t yet know if the bacteria are causing the cognitive decline or if changes in the body due to Parkinson’s allow these bacteria to grow.

“But our findings suggest they may play an active role in worsening symptoms.”

The study analysed 228 stool and saliva samples from people with Parkinson’s and a healthy control group.

The Parkinson’s participants were grouped by disease stage: those with mild cognitive impairment and those with dementia.

Scientists observed clear differences in the types and activity of bacteria across the groups.

In those with cognitive symptoms, the gut contained more harmful bacteria, many of which likely originated in the mouth.

This migration – known as oral-gut translocation – introduces oral bacteria into the gut, where they do not usually belong.

These bacteria were found to release toxins called virulence factors, which can damage gut tissue, increase inflammation, and may influence the brain via the gut-brain axis – the communication network between the digestive system and the brain.

Artificial intelligence was used to analyse the data, helping researchers identify bacterial species and functional changes not easily detected through traditional analysis.

The AI tools linked specific toxins to worsening cognition in Parkinson’s.

Dr Clasen said: “These toxins could be used as biological markers to identify patients at higher risk of dementia in Parkinson’s.

“In the future, they might also be targets for new treatments that protect the brain by changing the gut environment.”

The human microbiome – the collection of bacteria and other microorganisms in and on the body – plays a key role in many biological systems.

While the gut-brain axis has been widely studied, there has been less research on its link to cognitive decline in Parkinson’s.

The findings highlight the importance of oral hygiene and nutrition in Parkinson’s, especially as the disease progresses and patients may become more dependent on carers.