Mouth bacteria linked to increased head and neck cancer risk

More than a dozen bacterial species among the hundreds that live in people’s mouths have been linked to a collective 50% increased chance of developing head and neck squamous cell carcinoma (HNSCC), according to US researchers. Some of these microbes had previously been shown to contribute to periodontal disease — serious gum infections that can eat away at the jawbone and the soft tissues surrounding teeth.

Experts have long observed that those with poor oral health are statistically more vulnerable than those with healthier mouths to HNSCC, a group that includes the most common cancers of the mouth and throat. While small studies have tied some bacteria in these regions (known as the oral microbiome) to the cancers, the exact bacterial types most involved had until now remained unclear.

The research team, led by NYU Langone Health and its Perlmutter Cancer Center, analysed data from three ongoing investigations tracking 159,840 Americans from across the country to better understand how diet, lifestyle, medical history and many other factors are involved in cancer. Shortly after enrolling, participants rinsed with mouthwash, providing saliva samples that preserved the numbers and species of microbes for testing. Researchers then followed up for roughly 10–15 years to record any presence of tumours.

The investigators analysed bacterial and fungal DNA from the spit samples. Then they identified 236 patients who were diagnosed with HNSCC and compared the DNA of their oral microbes with that of 458 randomly selected study subjects who had remained cancer-free. The team accounted for factors known to play a role in developing such cancers, such as age, race, and how often subjects smoked cigarettes or drank alcohol.

Of the hundreds of different bacteria that are routinely found in the mouth, 13 species were shown to either raise or lower risk of HNSCC. Overall, this group was linked to a 30% greater likelihood of developing the cancers. In combination with five other species that are often seen in gum disease, the overall risk was increased by 50%.

“Our findings offer new insight into the relationship between the oral microbiome and head and neck cancers,” said lead author Dr Soyoung Kwak, a postdoctoral fellow at the NYU Grossman School of Medicine. “These bacteria may serve as biomarkers for experts to flag those at high risk.”

The researchers’ study, published in JAMA Oncology, is understood to be the largest and most detailed analysis of its kind to date. The authors have however emphasised that their work was designed to identify correlations between risk of cancer and certain bacteria in the mouth, but not to establish a direct cause-and-effect link; that will require further research.

“Now that we have identified key bacteria that may contribute to this disease, we next plan to explore the mechanisms that allow them to do so and in what ways we can best intervene,” said study co-senior author Professor Jiyoung Ahn, Associate Director for Population Research at the Perlmutter Cancer Center. Meanwhile, the current results offer yet another reason to keep up good oral hygiene habits.

“Brushing your teeth and flossing may not only help prevent periodontal disease, but also may protect against head and neck cancer,” concluded co-senior author Professor Richard B Hayes, a member of the Perlmutter Cancer Center.

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