Lung cancer in non-smokers: are mouth bugs to blame?

The role of the oral microbiome in a range of cancers is becoming increasingly clear.

The mouth microbiome may contribute to lung cancer risk, according to a major study of non-smokers.

Having a concentrated number of specific bacteria living in the mouth is linked to a higher risk of lung cancer among people who don’t smoke, according to a study of more than 100,000 people in Thorax.

Smoking isn’t responsible for every case of lung cancer, yet the study authors believe that factors such as secondhand smoke, air pollution and a family history of lung cancer can’t completely account for the roughly quarter of cases that occur in nonsmokers.

The mouth microbiome has been linked to a range of cancers, including head and neck, pancreas and the gullet, prompting Dr H Dean Hosgood, at the Albert Einstein College of Medicine in the Bronx, and colleagues to investigate whether it played a role in lung cancer too.

The team analysed data from the Shanghai Women’s Health Study and the Shanghai Men’s Health Study, which spanned 1996 to 2006.

A sample of mouth bacteria was taken from participants at the study outset, as was data about their medical history, diet and lifestyle, and other environmental or workplace health risk factors, and their health was tracked every couple of years during the study period.

The participants were lifelong non-smokers.

Dr Hosgood and his colleagues found that 90 women and 24 men developed lung cancer, and compared their cases to 114 matched controls without lung cancer.

The controls had a similar age, sex, level of education and family history of lung cancer.

What appeared to set the two groups apart was their mouth microbiome. Those who developed lung cancer were less likely to have a wide range of bacterial species living in their mouths, and more likely to have specific types of bacterial species. These problem species were Firmicutes species – most notably the lactobacillales order of microbes. Meanwhile, a high volume of Bacteroidetes and Spirochaetes species, such as Spirochaetia, appeared to be protective.

The link continued even among those without recent antibiotic use.

The mouth microbiome is less understood than the gut, Dr David Christiani, of Harvard University, wrote in an accompanying editorial.

“The breadth and depth of the oral microbiome is impressive: 1 mL of saliva contains 108 microbial cells and 700 distinct prokaryotic taxa,” he wrote. “There are rich communities such as bacteria, fungi, viruses, archaea and protists, among which about 54% are cultivatable and identified, 14% are cultivatable, but not identified, and 32% are not even cultivatable”

This finding adds to our growing understanding of the role these microbes play in a range of respiratory and other illnesses, he said, but raised several questions that still need to be answered.

“First, how stable is the human oral microbiome over time? Second, if the human oral microbiome varies over time, what determines that variability? Third, how does the ambient environment such as exposure to air pollutants, affect the oral (and lung) microbiome?”

While the study showed that the oral microbiome played a role in lung cancer, the authors warned that the population was restricted to one location and at one point in time.

Thorax, December 14