Killer smile

The bacteria and fungi that inhabit our mouths have been found to play an important role in the development of diseases, including cardiovascular disease, diabetes, rheumatoid arthritis, certain cancers and Alzheimer’s, reports Katrina Megget

The human mouth is home to more than 20bn microorganisms and, for the most part, this oral microbiome lives in a state of equilibrium; the friendly bacteria keeping the bad guys at bay. But sometimes this equilibrium is lost, leading to dysbiosis or an imbalance in the microbial community. The result is two of the most common diseases to affect humans: tooth decay and gum disease. According to the US Centers for Disease Control and Prevention, gum disease affects around 47% of the US adult population.

Evidence now suggests that an unhealthy or dysbiotic oral microbiome not only causes tooth decay and gum disease but also appears to have a role in the development and exacerbation of systemic diseases , a topic discussed at the virtual AAAS meeting in February. There are possible links between the oral microbiome and exacerbation of the inflammatory bowel diseases ulcerative colitis and Crohn’s disease, and also a relationship with some autoimmune diseases.

Research in February 2021 even suggests Covid-19 patients with gum disease are 31/2 times more likely to be admitted to intensive care, 41/2 times more likely to need a ventilator and almost nine times more likely to die, compared with Covid patients without gum disease (N. Marouf et al, Journal of Clinical Periodontology, doi: 10.1111/jcpe.13435).

‘The mouth is the gateway to the body,’ explains Purnima Kumar, professor of periodontology at Ohio State University, US. ‘There are so many species of bacteria in the mouth and the real estate is huge – it’s the size of the palm of your hand. But the oral microbiome isn’t Las Vegas – what happens in the mouth doesn’t stay in the mouth.’

The oral microbiome is established at birth and evolves over time. A healthy mouth microbiome is diverse with around 250-300 different species of bacteria that behave symbiotically for mutual benefit and maintain a check on pathogenic species from getting a foothold.

‘Generally, with disease, the diversity decreases and certain species emerge to become a higher portion of the overall microbiome. This occurs in tooth decay and periodontitis (gum disease),’ explains Jeffrey Ebersole, professor and associate dean for research at the University of Nevada Las Vegas’ School of Dental Medicine, US.

How the oral microbiome goes from healthy to unhealthy is not well understood. However, Kumar points out that it is a very resilient ecosystem and doesn’t fluctuate rapidly but it will be affected by diet, oral hygiene, lifestyle changes and disease. ‘This is a complex, highly evolved ecosystem like a rainforest or a coral reef,’ she says. ‘The climate in the mouth can influence the bacteria and when that climate isn’t functioning right, bad bacteria can bloom.’ This is seen in research that found cigarette smoking can change the amount and mix of bacteria in the oral microbiome within five years, while vaping can lead to dysbiosis within just six months, she says. Pregnancy too can alter the oral microbiome, yet six months post-pregnancy the original microbiome can return.

But it is the oral microbiome’s role in systemic disease that is creating a particular flurry of interest. The idea dates back more than a hundred years, but lost favour with scientists until recently. It is only in the last 30 years that research into the oral microbiome has ramped up. The result is increasingly robust evidence pointing to correlations and associations between an unhealthy oral microbiome, gum disease and various systemic diseases.

‘Substantial association data from around the globe have supported significant correlations between periodontitis and the severity of periodontitis and various systemic diseases,’ says Ebersole. ‘Since an unhealthy microbiome is directly linked to periodontitis, this changed microbiome is likely associated as well.’

Gum disease is triggered by bacterial colonies that form on the tooth surface near the gums. This stimulates inflammation in the gums, which can lead to gum disease and bone loss around the teeth. Gum inflammation not only creates an environment for pathogenic bacteria to colonise but also damages the epithelial tissue around the gums. This damage makes the tissue ‘leaky’ and allows the bacteria, along with inflammatory molecules like cytokines, to enter the systemic circulation where they can travel to distant organs to set up localised infection, injury and inflammation.

The concept has been corroborated by finding oral pathogenic bacteria – including Porphyromonas gingivalis, Treponema denticola, Fusobacterium nucleatum and Campylobacter rectus – in distant areas of the body including joint cavities, the placenta, pancreas, atheromatous plaques on artery walls, and the brain. Indeed, periodontitis has now been linked to 57 human non-communicable diseases, says Iain Chapple, professor of periodontology and consultant in restorative dentistry at the University of Birmingham, UK.

Chapple’s research has focused on rheumatoid arthritis (RA), and while proof of causality has not yet been established, Chapple says the oral pathogen P. gingivalis is known to produce the enzyme peptidylarginine deiminase or PAD. This enzyme catalyses the deimination of arginine residues into citrulline amino acids. While citrullination is important for neuronal development and chromatin remodelling, it is also upregulated during bacterial infection and inflammation. Citrullinated proteins are the target of the autoantibodies that cause RA, Chapple says.

‘It is not always the bacteria that are the issue per se, more the inflammation in the gums that results from those bacteria,’ Chapple explains. ‘What happens is the bugs stimulate immune cells called neutrophils, and about a third of all neutrophils when challenged, die by releasing their DNA in the form of neutrophil extracellular traps or NETs. The NETs are like a sticky spider’s web, designed to trap the bugs but are full of citrullinated proteins and they are thought to generate the auto-antibodies in the gums, rather than the bacteria directly.’

Research shows that P. gingivalis infection precedes the onset of RA and that autoantibodies targeted to citrullinated proteins are higher in people with periodontitis. ‘It is likely that gum disease is just one of many potentially causal factors for RA developing in some but not all people,’ Chapple says. ‘But we need some larger studies that treat periodontitis within RA patients and follow them for a year, minimum, ensuring the gum disease is treated to a good endpoint and kept stable. This may tell us if treating gum disease improves outcomes of RA – and if it does, then this could represent a non-drug-based therapy that may be very helpful in stabilising RA.’

P. gingivalis has also been implicated in the initiation or progression of Alzheimer’s disease, being found in cerebral spinal fluid and brain tissue. According to Mark Ryder, professor of periodontology at the University of California San Francisco, US, much research has focused on this area over the past 10-15 years, with particular attention paid to a class of cysteine protease enzymes produced by P. gingivalis called gingipains. ‘These enzymes break down healthy tissue for the bacteria’s nutrients, they can invade cells, are toxic to neurons, set up inflammation in the brain, may disturb sleep patterns and could play a multitude of roles in the initiation and progression of Alzheimer’s disease,’ Ryder says.

Scientists have shown the biology of the correlation between bacteria and disease in cultured brain cells and in animal models. For instance, mice orally infected with P. gingivalis had brains that were colonised with the bacteria and showed an increased production of the β-amyloid peptide, a component of amyloid plaques, which is a characteristic pathology of Alzheimer’s disease (V. Ilievski et al, Plos One, doi: 10.1371/journal.pone.0204941). Recent research has also identified the presence of gingipain enzymes in the brain of Alzheimer’s patients, with levels correlating with the aggregation of tau proteins, another pathology of Alzheimer’s disease (S. Dominy et al, Science Advances, 2019, 5(1):eaau3333).

Ryder was involved in a landmark study to design and synthesise small-molecule inhibitors targeting gingipains to see whether this blocked neurotoxicity (S. Dominy et al, Science Advances, 2019, 5(1):eaau3333]. The results showed that gingipain inhibition reduced the bacterial load of an established P. gingivalis brain infection, blocked β-amyloid production, reduced neuroinflammation and rescued neurons in the hippocampus, suggesting the drug could potentially treat Alzheimer’s disease. ‘The effect of an intervention will seal the deal that the oral microbiome is a major factor in the initiation and progression of Alzheimer’s disease; that it’s not just an association,’ Ryder says.

Indeed, the studies have been so positive, the inhibitor molecule atuzaginstat (COR388) has moved into clinical trials, led by biopharma firm Cortexyme. In small Phase 1 trials, the molecule was found safe and improved cognitive function in people with mild to moderate Alzheimer’s disease. The inhibitor is now in Phase 2/3 trials, with the results expected by the end of 2021. ‘If we have a targeted treatment we can use at the early stages of Alzheimer’s disease or in people who are more susceptible to the disease, then we can attack this disease earlier,’ Ryder says.

[The oral microbiome] is a complex, highly evolved ecosystem like a rainforest or a coral reef. The climate in the mouth can influence the bacteria and when that climate isn’t functioning right, bad bacteria can bloom.
Purnima Kumar professor of periodontology, Ohio State University, US

For some systemic diseases, however, things are not so clear cut. Diabetes, for instance, has a complex relationship with the oral microbiome. ‘Diabetes is a two-way street,’ explains Kumar. ‘Oral bacteria can react to the disease like a canary in a coal mine. In other words, diabetes can change the microbiome, which changes the trajectory of the disease and plays a role in making insulin management worse.’

Dana Graves, professor of periodontics and vice dean for scholarship and research at the University of Pennsylvania, US, calls the relationship bidirectional. He led a 2017 study in mice that showed diabetes caused a shift in the oral microbiome to increased pathogenicity and enhanced inflammation, compared with non-diabetic mice (E. Xiao et al, Cell Host & Microbe, 2017, 22(1):120). ‘Diabetes has been shown to increase inflammation in the mouth and it is likely that the increased inflammation alters the bacteria that predominates, as the growth of some bacteria is enhanced in an inflammatory environment,’ he says. But there is also evidence, he adds, that periodontal disease can worsen glycaemic control in people with Type 2 diabetes. A number of studies have shown that effective treatment of gum disease reduces blood glucose levels, improves glycaemic control and may decrease the amount of medication needed to control and manage diabetes. But as yet, the relationship remains a ‘chicken and egg’ question, with much to be deciphered.

So, given that as much as 50% of the global population has some form of gum disease, should we be worried? It’s not that simple. ‘I think the leap is too big to make a statement that the oral microbiome “causes” these systemic diseases,’ says Ebersole. ‘However, the literature is pretty robust that they are detected in these distant tissues and probably contribute to the disease symptoms.’

The thing to remember is these systemic diseases are multifactorial, adds Kumar. In diabetes, for instance, there are 15 or 16 risk factors that can lead to the development of the disease. ‘The oral microbiome is just one factor.’ Ebersole puts it another way: ‘If you remove all the other risk factors for these systemic diseases and a patient only had periodontitis, would they be susceptible to these systemic diseases? Probably not.’

That said, the consensus is that having a healthy mouth is the preferable option. ‘The bottom-line belief,’ says Ebersole, ‘is that the longer an unhealthy microbiome exists and the longer an individual enables this unhealthy microbiome through diet and behaviours, the more likely it will contribute to disease over time.’ But, he adds, the more the local microbial burden and gum inflammation is controlled through oral hygiene and dental visits, the lower the likelihood of aggravating systemic diseases.

Indeed, research with some conditions – including pneumonia, COPD, diabetes and cardiovascular disease – suggests treating the oral microbiome can decrease the incidence or lower the risk of disease. But, in general, there is a shortfall in conclusive research and still many unanswered questions. This includes understanding the feedback loop between disease and the oral microbiome to fully understanding the composition of the oral microbiome, the shift to dysbiosis and the roles specific bacteria play. More needs to be done to understand the gum disease process, as well as the relationship between severity and time frame of dysbiosis and systemic disease, and also the influence of host genetics.

‘There is a large amount of work to be done and each disease is different, and every person is different,’ Kumar says. ‘We’re really just scratching the surface looking at how the oral microbiome exacerbates disease or increases risk.’

Yet, the scope for new therapeutics is promising, either to directly treat or prevent systemic disease, as seen with the Alzheimer’s research, or to derive new approaches to reduce the development of dysbiosis and target specific oral bacteria. Probiotics, which would introduce friendly bacteria to the mouth, could be a dead-end because their presence in the mouth is short-lived as they easily wash away, but prebiotics, compounds that induce the growth of beneficial bacteria, could hold promise, both Ryder and Kumar believe. Indeed, Kumar is studying black raspberries as a potential prebiotic in modulating the microbiome of smokers.

It is becoming clear that periodontitis is not just a dental disease. Kumar notes there is an increasing public health focus on this, such as the EU push to improve oral health in prenatal women. But she says more needs to be done to provide an integrated system with holistic care for patients across different diseases.

The research may still be in its infancy, but joining up the dots is tantalising. The bottom-line, however, says Ryder, ‘this all just emphasises the importance of having a healthy mouth’.