Poor dental health and gum disease may be linked to Alzheimer's disease, a study suggests.
Brains of deceased dementia patients were found to contain signs of Porphyromonas gingivalis, the bug responsible for unhealthy gums.
Scientists believe when the bacteria reach the brain they trigger an immune response that can lead to the death of neurons.
The process could help drive the changes that are typical of Alzheimer's disease, causing symptoms of confusion and memory loss.
Scientists at the University of Central Lancashire (UCLan) studied 10 brain samples from patients who died with dementia. They were compared with the same number of samples from non-dementia sufferers.
Professor StJohn Crean, dean of the School of Medicine & Dentistry at the university, said: "This new research indicates a possible association between gum disease and individuals who may be susceptible to developing Alzheimer's disease, if exposed to the appropriate trigger.
"Research currently under way at UCLan is playing an active role in exploring this link, but it remains to be proven whether poor dental hygiene can lead to dementia in healthy people, which obviously could have significant implications for the population as a whole. It is also likely that these bacteria could make the existing disease condition worse."
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The findings are reported in the Journal of Alzheimer's Disease. More recent work by the team has confirmed in animals that P gingivalis in the mouth finds its way to the brain once gum disease becomes established.
Lead researcher Dr Sim Singhrao, also from UCLan, said: "We are working on the theory that when the brain is repeatedly exposed to bacteria and/or their debris from our gums, subsequent immune responses may lead to nerve cell death and possibly memory loss.
"Continued visits to dental hygiene professionals throughout one's life may be more important than currently envisaged with inferences for health outside of the mouth only."
Dr Simon Ridley, from the charity Alzheimer's Research UK, said: "This is an early study with a very small number of samples that observed bacteria linked to gum disease in the brains of people with Alzheimer's.
"We don't know whether the presence of these bacteria in the brain contributes to the disease and further research will be needed to investigate this. It is possible that reduced oral hygiene and therefore P gingivalis infection could be a consequence of later stage Alzheimer's, rather than a cause.
"Other studies have suggested that infections, including oral infections, could be linked to Alzheimer's and there is ongoing research in this area. It will be important for future studies to consider looking back at dental records, to correlate these kinds of observations with the level of oral hygiene during life.
"We know that there are likely to be many risk factors for Alzheimer's and we need to investigate these in more detail to help develop new preventions or treatments. While a causal link between poor oral health and Alzheimer's is still unclear, people who are worried about their dental health should still visit their dentist regularly."
Dr Alison Cook, from the Alzheimer's Society, said: "There have been a number of studies looking at the link between dementia and inflammation caused by factors including poor dental health, but this is not yet fully understood. This small study suggests that we need more research into this important area.
"The best way to reduce your risk of dementia is to lead a healthy lifestyle. Enjoy a balanced Mediterranean diet rich in fruit and vegetables, oily fish and even the occasional glass of red wine, take regular exercise and don't smoke. Of course if people are worried, it never hurts to reach for the tooth brush twice a day."