How do non-smokers develop lung cancer? Mouth bacteria may play role, study suggests

TORONTO — Even those who have never smoked a cigarette in their lives can develop lung cancer — and new research suggests that one mechanism behind this could be specific types of mouth bacteria.

A paper published Monday in the scientific journal Thorax which looked at non-smokers found that the chances of developing lung cancer might be linked to the type and amount of mouth bacteria a person has.

According to the researchers, non-smokers make up around one fourth of those who develop lung cancer, and known risk factors such as second-hand smoke and family history of cancer can’t fully explain this.

The research accessed two different study groups — the Shanghai Women’s Health Study and the Shanghai Men’s Health Study, which involved more than 135,000 people whose health was monitored every few years between 1996 and 2006.

Researchers only looked at lifelong non-smokers in the study groups who had no cancer at the start or within two years of starting the study.

When they were entered into these studies, participants provided a profile of the bacteria in their mouth with an oral rinse. Information was also provided on their family’s medical history and other factors that could influence their health.

Researchers found that out of the non-smokers, 90 of the women and 24 of the men who participated in these large study groups went on to develop lung cancer at some point within approximately seven years.

To compare the bacteria of this group with another, researchers looked at mouth rinse samples from 114 non-smokers with matching ages and genders who had not developed lung cancer.

“This comparison group didn’t have lung cancer but they had similar levels of education and family histories of lung cancer,” a press release states.

Different types and variety of mouth bacteria have been associated with risks for other types of cancer before, such as cancers that can be in the head, neck and throat, so researchers wanted to know if it could affect lung cancer as well, as air passes through the mouth before it reaches the lungs.

The two sets of mouth bacteria samples — from non-smokers with lung cancer and the control group of non-smokers without — showed there were significant differences between the microbiomes of the two groups.

There were two factors that were relevant when it came to the bacteria: variety and kind of bacteria.

Having a wider number of different species of bacteria was associated with a lower risk of developing lung cancer. In terms of specific bacterial species, having a higher volume of Bacteroidetes and Spirochaetes — two phyla, or categories, of bacteria — was associated with lower risk of lung cancer. Bacteroidetes are frequently found in the gastrointestinal tract, and Spirochaetes are a type of bacteria that are spiral-shaped.

However, having a higher volume of Firmicutes species of bacteria in the mouth was associated with a heightened risk for lung cancer. Within the Firmicutes phylum, a higher number of the Lactobacillales order specifically was associated with a heightened risk — a type of bacteria better known as lactic acid bacteria, which is usually found in decomposing plants and dairy products.

Previous research, the study pointed out, had found that an abundance of Firmicutes found in fluid squirted into a segment of the lungs and then suctioned back out during a procedure called bronchoscopy had also been associated with lung cancer.

The study noted that when looking only at the limited number of men in the study, “only an increased abundance of Spirochaetia was associated with risk of lung cancer.”

The researchers clarified that this is an observational study, which means they weren’t looking to explain the phenomenon, only confirm that this bacterial association exists. Why specific mouth bacteria may increase the risk of getting lung cancer is a question that needs more research.

A scientific editorial also published in Thorax, penned by David D. Christiani, a Harvard professor, commented on this new research, saying it raises many questions.

“In terms of questions being provoked by the study, it remains unclear whether the oral microbiome as measured in this (and other) epidemiological study represents a causative agent or only a marker of disease or immune activity,” Christiani wrote. “If it is the former, then it will be important to understand whether the oral microbiome actually seeds the lung microbiome and thus acts locally.

“The oral cavity and nasopharynx are the foyers to the lung and further understanding of the functional aspects of the microbiome in these compartments will surely help shed light on disease pathogenesis and disease modification.”

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