Mounting evidence suggests that the microbes in our mouths could be putting us at risk of certain cancers, as well as affecting our prognosis if we do develop them, but the relationship isn’t straightforward.
Second only to the gut, the mouth is home to a diverse microbial community, with more than 700 species of bacteria alone colonising our teeth, tongues and soft tissues.
Over the past decade, research has increasingly linked gum disease and the microbes that arise through poor oral hygiene to various conditions, including Alzheimer’s disease and arthritis. Evidence is also implicating the oral microbiome in head and neck cancers, Ěý˛ą˛Ô»ĺ , including the mouth, throat, sinuses and salivary glands.
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Until recently, most of these studies were small, and they only flagged certain groups of bacteria in people who already had cancer, making it unclear whether they were drivers of the condition or merely correlative passengers.
But in September, researchers led by at the New York University Grossman School of Medicine edged closer to clearing up that uncertainty. prospectively tracking the health of 159,840 people in the US. Saliva samples were taken at the start of the studies.
Around 10 to 15 years later, the development of head and neck squamous cell carcinoma – which affects the mucous membranes lining the mouth, nose and throat – was correlated with the presence of up to 13 bacterial species in the saliva samples. These species hadn’t previously been linked to the condition.
The researchers also detected a moderate association with the so-called orange and red complex bacteria, which have a strong link to gum disease. Together, the presence of these bacteria was associated with a significantly increased risk of head and neck cancer.
The study wasn’t designed to establish cause and effect, but it still provides some of the strongest evidence yet that certain bacteria may be associated with the development of cancer.
“I think this is another reason to keep up good oral hygiene,” says team member , also at NYU Grossman School of Medicine.
Other research groups have been unpicking the mechanisms through which certain oral bacteria might influence our cancer susceptibility. Earlier this year, at the Chinese University of Hong Kong and his colleagues oral cavity tumours, but not healthy tissue. Further experiments suggested that they correlate with the upregulation of various genes involved in cell proliferation, migration and adhesion – pathways that are often dysregulated in cancer.
“Although we don’t have the data to support a causal linkage between bacteria and cancer at this point, our study suggests that bacteria do play a role in these cancers and seem to drive their development – particularly, we think, at an early stage,” says Chan.
One species identified in both studies is Fusobacterium nucleatum, a key member of the red complex bacteria. Earlier lab-based research by at the Hadassah-Hebrew University in Jerusalem and his colleagues found that .
This could provide tumours almost free rein to grow, says at King’s College London, who is investigating the role of microbes in improving cancer treatment.
the relationship isn’t clear-cut. His team found that people with head and neck cancer who have higher levels of F. nucleatum actually have a better prognosis than those with lower levels. Follow-up experiments in labs also identified a large reduction in the number of viable cancer cells after they were infected with F. nucleatum bacteria. “They essentially melt head and neck cancer cells,” he says.
Referring to the research by Hayes and Kwak’s team, Reis Ferreira says that while this was a very large study, it doesn’t prove that red or orange complex bacteria cause cancer. “It could just be an association, because these bacteria thrive in environments where there is a lot of inflammation, and inflammation is a known risk factor for cancer.”
But understanding these relationships is clearly important. “Head and neck cancer is an underserved disease, and the treatment hasn’t really evolved very much, so there is a lot of room for improvement,” he says.
One hope is that some of these bacterial profiles could be used, perhaps in combination with lifestyle information, to classify people into high or low-risk groups for developing head and neck cancer. “Those in high-risk categories could then be considered for more careful oral examination for cancer,” says Hayes.
Altering people’s oral microbiome might also affect their outcomes, says Chan. “We know from melanoma studies that manipulating the gut microbiome can improve their response to immunotherapy. So, through the microbiome, could we also influence treatment outcomes for these patients with head and neck cancer?”
