Would you willingly put a stranger’s saliva in your mouth if it could improve your health?
As unappealing as it sounds, volunteers are doing exactly that in clinical trials investigating oral microbiome transplants. The treatment involves transferring bacteria from a healthy person’s mouth to someone experiencing oral health problems, in the hope of restoring a healthier bacterial balance.
Current trials are exploring whether the approach could help treat tooth decay, halitosis, or bad breath, and oral mucositis — a painful condition in which the lining of the mouth becomes inflamed after cancer treatments such as chemotherapy and radiotherapy.
Some experts believe the relatively simple procedure could eventually have wider benefits, including reducing the risk of serious illnesses such as cancer.
The idea follows growing interest in the gut microbiome — the community of organisms living in the intestines that plays an important role in overall health. Faecal transplants, which involve placing stool from a donor with a healthy gut microbiome into a recipient’s bowel, are already being investigated as treatments for conditions including irritable bowel syndrome.
Scientists are now turning their attention to the microorganisms living in the mouth. ‘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.
Beneficial bacteria coat the teeth, gums, tongue and inner cheeks. They help create a protective barrier that blocks harmful pathogens, limiting their ability to multiply in mouth tissue, enter the bloodstream and travel to vital organs. However, emerging research suggests that, as with the gut microbiome, disruption to this balance could contribute to widespread inflammation and diseases such as cancer.
Research published in the journal RMD Open in July even reported a possible association between an imbalanced oral microbiome and osteoarthritis.
Recent studies suggest that, like disruption to gut bacteria, an unhealthy oral microbiome may be associated with widespread inflammation and conditions including cancer.
Researchers from China, Australia and the UK analysed saliva samples from 52 people with hand arthritis, a condition thought to affect nearly 1.5 million people in Britain. Their results were compared with samples from healthy participants.
People with hand arthritis had significantly higher levels of Trichococcus, a bacterial species commonly found in sewage that can colonise the mouth after contact with contaminated surfaces.
The study found that greater amounts of the organism were associated with more severe joint pain and inflammation. One theory is that the immune system attacks the bacteria in the mouth, causing low-level, persistent inflammation that may contribute to cartilage breakdown in the joint.
This is not the only research linking an unhealthy oral microbiome — which can develop through poor dental hygiene and excessive mouthwash use — with disease elsewhere in the body.
A study published in BMC Microbiology in July found that people with early-onset bowel cancer had noticeably different oral bacterial profiles from healthy participants.
They had higher levels of certain bacteria and, importantly, less overall bacterial diversity. Greater diversity in either the oral or gut microbiome is generally associated with better health.
It remains unclear whether the bacterial imbalance contributes to cancer or whether cancer causes the changes in the microbiome. Either way, researchers said saliva testing could eventually offer a straightforward way to screen for early-onset bowel cancer — diagnosed before the age of 50, and rising in prevalence — before symptoms appear. Scientists at King’s College London have also identified significant differences between the oral microbiomes of healthy people and those with Parkinson’s disease, which affects around 160,000 people in the UK.
The findings, published in Gut Microbes last year, showed that people with Parkinson’s who had developed dementia — a complication affecting up to 50 per cent of patients — carried higher levels of potentially harmful bacteria than patients without dementia and healthy participants.
The bacteria had moved from the mouth into the gut, where they prompted the release of toxins capable of damaging the stomach, driving widespread inflammation and potentially contributing to dementia-like damage in the brain.
Lead researcher Frederick Clasen said: ‘We don’t yet know whether the bacteria are causing cognitive decline or whether changes linked to Parkinson’s allow these organisms to flourish.
‘However, our findings suggest they could play an active part in worsening symptoms.’
‘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
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.
