Would you put a stranger’s saliva in your mouth as a medical treatment?
Gross as it may sound, that’s exactly what some brave volunteers are now doing as part of clinical trials to see if oral microbiome transplants – transferring bacteria from the mouths of healthy people into those with problems – are a new medical breakthrough.
The trials are currently looking at tackling tooth decay, halitosis (bad breath) and oral mucositis – a painful condition where the inside of the mouth becomes sore and inflamed after cancer treatments such as chemotherapy and radiotherapy.
But some experts hope this simple procedure may one day help reduce the risk of more serious diseases such as cancer.
Many of us are familiar with the gut microbiome – the intestinal ‘soup’ of organisms that plays a vital role in our overall health. And how faecal transplants, where faeces from a donor with a healthy gut microbiome are inserted into a recipient’s bowel, are being tested as treatments for conditions such as irritable bowel syndrome.
But now scientists are examining the organisms that populate our mouths as a way to treat disease. ‘The oral microbiome could prove to be just as important as the gut microbiome,’ says Dr Lindsey Edwards, a microbiome scientist at King’s College London.
‘Friendly’ bacteria cover the surface of our teeth, gums, tongue and inside of the cheek, helping form a barrier to keep out harmful pathogens and stop them from replicating in the mouth tissues, entering the bloodstream and circulating to vital organs. But recent studies suggest that, just like with gut bacteria, damage to the balance of the oral microbiome could be linked to widespread inflammation and conditions such as cancer.
In July, research published in the journal RMD Open revealed an imbalanced oral microbiome may even be linked with osteoarthritis.
Recent studies suggest that, just like with gut bacteria, damage to the balance of the oral microbiome could be linked to widespread inflammation and conditions such as cancer
Researchers from China, Australia and the UK studied saliva samples from 52 people with hand arthritis, thought to affect nearly 1.5 million Britons. They compared the results with saliva samples from healthy people.
Those with hand arthritis had significantly higher levels of a bacterial species called Trichococcus, a bug commonly found in sewage that can colonise the mouth if picked up from contaminated surfaces.
The higher the levels of this organism, the more severe the joint pain and inflammation, the study found. It’s thought the immune system may attack the bugs in the mouth, leading to low-level, chronic inflammation that triggers cartilage breakdown in the joint.
But it’s not the only study to show a ‘bad’ oral microbiome (which can develop due to poor dental hygiene and overuse of mouthwash) may be connected to illnesses.
In research published in BMC Microbiology in July, patients with early-onset bowel cancer were found to have very different oral bacteria profiles from their healthy peers.
They had higher concentrations of some bacteria and, crucially, a smaller variety of bugs overall (the more diverse the oral or gut microbiome, the healthier they are).
It is unclear whether bacterial imbalance fuels the cancer or if the cancer drives bacterial changes. But either way, researchers said checking saliva for a bacterial profile might be an easy way of screening for early-onset bowel cancer (where it occurs before the age of 50 – such cases are on the rise) before any symptoms. Scientists at King’s College London have also found marked differences between the oral microbiomes of healthy people and patients with Parkinson’s disease, which affects some 160,000 people in the UK.
The results, published in Gut Microbes last year, showed those with Parkinson’s who had developed dementia as a result of their condition (which affects up to 50 per cent of people with Parkinson’s) had higher levels of ‘harmful’ bacteria than patients without dementia or healthy peers.
The bugs had travelled from the mouth to the gut, where they triggered the release of toxins that can damage the stomach, cause widespread inflammation and possibly lead to dementia-like damage to the brain.
Lead researcher Frederick Clasen said: ‘We don’t know yet if the bacteria are causing the cognitive decline, or whether changes in the body due to Parkinson’s allow these bacteria to grow.
‘But our results suggest they may play an active role in worsening symptoms.’
Dr Edwards says health problems tend to arise when ‘bad’ bacteria in the mouth pass down into the gut – where they don’t belong. If they survive the acidic conditions of the stomach they can set up home in the intestines, disrupting the normal balance of gut microbes and triggering a harmful immune response that could lead to ill health. Meanwhile, if harmful mouth bacteria get into the bloodstream (through bleeding gums, for example) they can travel to the heart and attack the lining of the four chambers, causing a potentially fatal condition, infective endocarditis.
So could transplanting ‘healthy’ saliva help to protect against bacteria-related diseases? A handful of trials are under way to investigate this.
One, at the University of Washington in the US, is looking at whether it could be a cure for halitosis, which affects one in four Britons. It’s typically caused when ‘bad’ bacteria break down food in the mouth, producing foul odours.
The treatment involves taking a swab from the donor’s mouth, extracting micro-organisms from it and adding them to a saline solution – the recipient is then given this solution to use as a mouthwash. A more concentrated solution can be injected directly into the gums.
The theory is the donated bugs will crowd out the ‘bad’ bacteria that cause bad breath.
Four patients have so far been given saliva transplants, and results of the full trial are expected later this year.
And a team at Adelaide University in Australia is exploring whether transplants can ward off tooth decay. During lab tests, they identified 250 types of ‘good’ bacteria in the mouth that, when transplanted, can help to prevent cavities by outnumbering ‘harmful’ bacteria that erode tooth enamel.
It’s early days but Dr Edwards is convinced the oral microbiome’s role in medicine is only going to get bigger.
‘As more research is conducted, it’ll be equally crucial [as our gut microbiome] to our health,’ she says.
How to look after the ‘good’ bacteria in your mouth
Avoid alcohol-based mouthwashes: Regular use of these kills off ‘good’ mouth bacteria (such as actinobacteria, crucial for helping regulate blood pressure) and allows ‘bad’ bacteria to flourish (including Fusobacterium nucleatum and Streptococcus anginosus, linked to gum disease and bowel cancer), reported the Journal of Medical Microbiology in 2024.
Floss daily: It shifts sticky plaque from between your teeth where ‘harmful’ bacteria flourish, forming thick layers. ‘Brushing and flossing daily is one of the best things you can do,’ says Dr Lindsey Edwards, a microbiome scientist at King’s College London.
Stay hydrated: Sipping water rinses away food particles and sugars that can stick to your teeth and feed ‘harmful’ bacteria.
Eat spinach: Leafy greens are rich in nitrates, plant compounds that act as prebiotics, feeding ‘good’ bacteria in the mouth.
Cut down on alcohol: Studies show just a couple of drinks a day can disrupt the bacterial balance in the mouth, as the toxins wipe out the beneficial bugs.
