Gum disease and heart health: how are they connected?

Is it healthy to have bacteria in your mouth?

“It’s completely normal and healthy to have bacteria in your mouth. But what we don’t understand is how they can shift from being completely harmless ‘good’ bacteria, to ones that cause gum disease and are linked to heart attacks,” says Dr Karolin Hijazi, at the University of Aberdeen.

You might have heard of the ‘human microbiome’ before, referring to the many microbes – mostly bacteria, but also fungi and viruses – that live on or in the human body. It’s been estimated that the average human body contains about 30 trillion human cells, but these are outnumbered by the 38 trillion bacteria in us.

The microbes in our body are often talked about in terms of gut health, and it’s true that many of them live in our gut. But there’s growing evidence that just as many, or more, live in our mouths, where around 700 different species of bacteria have been identified.

In general, this isn’t a bad thing. Some of these bacteria have been found to help break down food and limit the growth of disease-causing bacteria.

But bacteria in the mouth can also turn bad. They can infect the gums, causing inflammation and bleeding. If left untreated, this can cause inflammation of the parts which connect our teeth to the underlying bone.

Gum disease can lead to tooth loss and has also been linked to an increased risk of developing heart or blood vessel problems. But we still don’t really understand how gum disease and cardiovascular disease are connected.

• Find out what you can do to prevent gum disease.

How do bacteria ‘switch’ from good to bad?

With the help of BHF funding, Dr Hijazi wants to advance our knowledge in this area. Having first developed an interest in microbiology during her training as a dentist, she now leads a research group at the University of Aberdeen focused on understanding how microbes interact with mucosal surfaces – parts of our body that form a barrier between the internal and external environment, such as the lining of the mouth.

She says: “I’m very interested in how bacteria can colonise the body, and in general be harmless. But under certain conditions, they can switch on genes that help them to do things like get into the blood stream or ‘hijack’ our immune system, causing inflammation.”

In this BHF-funded project, Dr Hijazi and her team are looking at a type of bacteria called Porphyromonas gingivalis, or P gingivalis for short (gingivalis means ‘out of the gum’). It’s known to be involved in the development of gum disease, but has also been linked to coronary artery disease and heart attacks.

About 20 years ago, it was shown in mice that being infected with this bacterium speeds up atherosclerosis – the process of fatty material building up in the arteries. We don’t know exactly why this happens. But it’s likely to be caused by the bacteria’s effects on the immune system, leading to the release of chemical signals that cause inflammation. Inflammation is a normal part of the immune response to infection or injury, but when it becomes long-term, it can lead to atherosclerosis.

People with higher levels of these bacteria in their mouths tended to have more damage to their heart

The team recently carried out a study of 160 people admitted to Aberdeen Royal Infirmary with a heart attack. About two-thirds of the people involved had a severe gum infection.

They also looked at levels of bacterial genes involved in the switch from the bacteria living harmlessly in the mouth to becoming ‘bad’ and causing inflammation. Levels of these molecules were also increased in people with more severe heart damage and coronary artery disease.

In this new project, the team want to look into how exactly P gingivalis switches on its ‘bad’ genes, and whether this differs in people who have had, or are at a higher risk of, a heart attack.

Studying people who’ve had heart attacks

Dr Hijazi and her team will be recruiting people who have recently had a heart attack, people with stable angina, and people without coronary artery disease.

Participants will have a dental exam and samples taken from their mouth, so the researchers can analyse what is going on inside the P gingivalis they find there. The research team want to see if there are epigenetic variants of this bacterium linked to having a heart attack, and other markers linked with worse outcomes after a heart attack. They also want to better understand how this variation gives the bacteria disease-causing properties, like being able to change immune responses.

It really could open new horizons in our understanding of how the trillions of microbes in our bodies can affect our health

“Finding a connection between the epigenetic status of P gingivalis and heart and blood vessel health may pave the way for more research to see if the same is true for other bacteria found in our mouth. Most importantly, we can see if we can intervene to improve things. For example, does treating gum disease alter the epigenetic make-up of our oral bacteria? And does doing that reduce the risk of having a heart attack?”

“More broadly, if we can show this mechanism in the context of gum disease and the heart, it could open a whole range of new avenues to look into in other diseases too. Could a similar way of switching genes on or off exist in gut bacteria? Could that help explain the links between our gut health and conditions like rheumatoid arthritis or Parkinson’s disease? It really could open new horizons in our understanding of how the trillions of microbes in our bodies can affect our health."

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