Variations in oral bacteria linked with future risk of lung cancer in people who have never smoked, finds study

Lung cancer is the most common cause of cancer-related death among men and women worldwide. While a huge proportion of lung cancer cases occur due to tobacco use and smoking, a margin of this global disease also occurs in people who have never smoked in their lives. A 2007 study published in Nature Reviews Cancer suggests that around 25 percent of lung cancer cases occur in never-smokers and has a marked gender bias as it occurs more in women.

However, known lung cancer risk factors like secondhand tobacco smoke, radon exposure, household air pollution, outdoor air pollution, and a family history of lung cancer do not account for this disease burden among never-smokers completely. A new study published in the journal Thorax suggests that diversity variations in oral microbes may be associated with the risk of lung cancer among never-smokers.

Oral microbiome and lung diseases

This link between oral bacteria and lung cancer may not be that easily apparent but it’s based on an emerging scientific consensus that the oral microbiome - the collective genome of microorganisms that naturally reside in the oral cavity - has a huge role to play in lung health.

An editorial piece in the current issue of Thorax, where this above-mentioned study is published, explains that unlike the gut microbiome, the oral microbiome’s role in human health and disease incidence is not that well-known. But since the oral cavity provides the entry point for the gastrointestinal tract and is immediately linked to the respiratory tract as well, the health implications of the oral microbiome are relevant to multiple body systems.

A study published in the Journal of Oral Microbiology in 2019 indicates that increased amounts of oral bacteria can be found in the gut of people with inflammatory bowel disease (IBD), HIV, liver cirrhosis, colon cancer, gastroesophageal reflux disease (GERD), and alcoholism.

This suggests that the oral microbiome affects the health of the gut microbiome. Similarly, a study published in Current Oral Health Reports in early 2020 suggests that the oral microbiome affects the lung microbiome, and the dysbiosis it may cause could lead to pneumonia, cystic fibrosis, chronic obstructive pulmonary disease (COPD), and lung cancer. The recently published study in Thorax explores this link between the oral microbiome and lung cancer in never-smokers.

Oral bacteria and lung cancer in never-smokers

This case-control observational study collected data from two prospective cohort studies called the Shanghai Women’s Health Study (SWHS) and the Shanghai Men’s Health Study (SMHS). The 74,941 participants from SWHS were enrolled between 1996 and 2000, while the 61,480 SMHS ones were recruited between 2002 and 2006.

All participants had no cancer at the beginning of the study, but their cancer status was recorded via in-person follow-up surveys every 2-3 years. Of all these participants, 114 (90 from SWHS and 24 from SMHS) lifetime never-smokers developed lung cancer and were therefore selected for this study.

Each of the 114 participants donated a mouth rinse sample at the start of the study, all of which were matched and compared with samples from 114 healthy control subjects who had the same parameters like sex, age, never smoking, family history of lung cancer, etc. All the participants and controls were followed up on for an average of 13.9 years.

The researchers used a method called Microbiome Regression-Based Kernel Association Test (MiRKAT) to evaluate the association between cancer risk and microbiome diversity (which includes different types of ecological diversity called alpha, beta, and gamma).

A study in oral microbiome diversity

They found that the participants who developed lung cancer had decreased oral microbiome alpha diversity, meaning that having a wider range of oral microbiota indicates a reduced risk of lung cancer.

The researchers also found that never-smokers who had an increased abundance of certain types of microbes called Bacteriodetes and Spirochaetes phyla also had a reduced risk of lung cancer, while an abundance of Firmicutes phylum was found to increase the risk of lung cancer.

These findings highlight not only the fact that the oral microbiome has definitive associations with the risk of lung cancer in non-smokers but also that variations and abundance in just a handful of microbial families can alter this risk status by a lot. While this study was limited and could not establish precise risk ratios, its findings suggest that further research into this link between the oral microbiome and lung cancer is definitely needed.

For more information, read our article on Lung cancer.

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