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Oral microbiome becoming prime focus of research over links to disease beyond the mouth

Keeping your teeth and gums healthy could have far-reaching impacts reducing risks associated with diabetes, dementia, heart disease and some cancers

Tooth decay and gum disease are common problems that almost everyone has to deal with at some stage in their life. But, did you know that severe gum disease (periodontitis) is linked to several other conditions including diabetes, dementia, heart disease and some cancers?

As dentists and microbiologists track the journey of harmful bacteria from the mouth to other parts of the body, their attention is turning to how promoting a balanced oral microbiome might also be key to lessening the severity of some chronic illnesses.

It is estimated that about 700 species of bacteria, viruses and fungi live in each person’s unique oral microbiome.

“The oral microbiome is very diverse. The gums, teeth and tongue all have their own microbiome which are strongly influenced by sanitisers [toothpaste, mouth washes, etc] as well as the food you eat,” explains Prof Paul Ross, director of the APC Microbiome at University College Cork (UCC).


Prof Edward Lynch, former principal director of biomedical and clinical research at the School of Dental Medicine at the University of Nevada in the United States, has been studying how microorganisms in the mouth can lead to inflammation in various parts of the body for more than 30 years.

“Dentists are learning more and more about the links between various bacteria in the mouth – such as Porphyromonas gingivalis and Fusobacterium nucleatum – to other conditions. Studies have found that P. gingivalis is able to cross the blood-brain barrier and is found in patients with Alzheimer’s disease,” says Lynch, who now lives in Schull, Co Cork. He adds that studies have also found P. gingivalis in oral and digestive cancers and in arteriosclerosis, the fatty plaque deposits in the arteries that can cause heart disease and strokes.

In 2014, Lynch was the co-author of a study which looked at how patients with diabetes had a threefold to fourfold increased prevalence of severe periodontitis and that the risk of heart disease, stroke and early death were all more likely in patients with diabetes type one and two when periodontitis was present. The study authors concluded that good oral health should be an important part of diabetes care and management.

Anthropologists have analysed DNA with the aim of reconstructing a history of the human oral microbiome to chart changes from hunter-gatherers to farmers, following the industrial revolution and after the second World War

Dr Fergus Shanahan, the founder and former director of APC Microbiome Ireland at UCC, says various studies showing the presence of oral bacteria such as P. gingivalis and F. nucleatum in other parts of the body doesn’t necessarily mean that these bacteria cause the disease.

“It may well be that Alzheimer’s plaques in the brain are susceptible to being seeded [colonised] by bacteria that get into the bloodstream from diseased gums. Normally, such bacteria would be eliminated by the immune system but occasionally, and particularly in elderly people, they succeed in colonising areas,” he says.

Regardless of what might cause Alzheimer’s, he says that the evidence suggests that P. gingivalis may aggravate the disease. “It is also noteworthy that people with years of cognitive decline and Alzheimer’s disease are more likely to neglect their teeth and oral hygiene and are more like to have periodontal disease. Thus the problem may postdate rather than pre-date the onset of Alzheimer’s disease.”

Dr Amel Sami, a dentist who did her PhD at UCC on the links between chewing tobacco and some cancers, says the relationship between P. gingivalis and Alzheimer’s disease is a major area of research.

“This possible connection should shift our approach to Alzheimer’s research in the coming decade to help us recognise further how the oral microbiome could be a potential player influencing the onset and progression of this devastating neurodegenerative disorder,” Sami says.

Shanahan adds that many forms of chronic inflammation in the body (eg inflammatory bowel disease and some types of heart disease) can also be seeded with bacteria from the mouth. And some studies have already looked at the role of the gut microbiome in neurological disorders including Alzheimer’s disease.

But with such academic and public fascination with gut microbiome research, is it likely that interest in the oral microbiome will follow?

A recent article in New Scientist magazine reported that knowledge of the oral microbiome isn’t new and that in 1891, American dentist, Willoughby D Miller first proposed that bacteria could leave the mouth and travel to others parts of the body to cause disease.

Frank Billings, co-founder of the Institute of Medicine of Chicago, later speculated that tooth infections may be the cause of rheumatoid arthritis, another chronic inflammatory condition. More recently, anthropologists have analysed DNA with the aim of reconstructing a history of the human oral microbiome to chart changes from hunter-gatherers to farmers, following the industrial revolution and after the second World War.

The New Scientist article also asked whether oral microbiome transplants might be on the horizon as a way of repopulating the oral microbiome with health-enhancing microbes.

Lynch cites one study in which 18 beagle dogs with gum disease were divided into test and control groups with the former group inoculated with the microorganisms of a younger healthy dog without gum disease. “After two weeks, there was a noticeable shift in the microorganisms present in the test group of dogs but at 10 weeks this shift had gone and there was no significant difference between the test and control group of dogs,” he notes.

Shanahan advocates caution on the promotion of oral microbiome transplants, though he is not against the principle of microbial transplants in non-gut body sites. “Firstly, while faecal transplants are now more widely accepted for C. difficile associated disease, they are far less effective and ineffective in many cases for other conditions, regardless of their growing popularity ... Much more research is needed and the mechanisms need to be defined,” he adds.

While the idea of oral microbiome transplant is appealing, Sami points out that the oral microbiome is extremely diverse and manipulating it would require “complex dynamics to avoid unintended consequences”.

As in all things, prevention is preferable to cure. “Modern diets are not what cavemen ate and we have a diverse microflora with complex sugars,” Lynch says. The oral health of our ancient ancestors was generally speaking much better due to their relatively sugar-free diets. His advice for people nowadays is like that which you’ve heard before from your dentist, if you’re eating sugary foods, do it all at once rather than grazing on it for a longer time.

“Eat the entire bar of chocolate at one time and then use a good oral hygiene product directly afterwards. This will lead to fewer problems with your teeth and gums,” says Lynch, who has recently developed new oral hygiene products with 33 active ingredients which will protect teeth and gums while keeping the oral microbiome intact. “The aim of these products is to substitute good microorganisms into the oral microbiome and use materials that are in the teeth already as well as a probiotic,” he says.

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Sylvia Thompson

Sylvia Thompson

Sylvia Thompson, a contributor to The Irish Times, writes about health, heritage and the environment