Smokers' mouths have lower levels of the bacteria that break down smoking-related toxins.
Cigarette smoking is the number one cause of preventable disease and mortality in the US, leading to 480,000 deaths annually, or 20% of all deaths.
Over 16 million people live with a smoking-related illness in the US, according to figures from the Centers for Disease Control and Prevention (CDC).
In 2014, the CDC estimated that 16.8% of Americans aged 18 years and over were cigarette smokers, or around 40 million adults.
There are around 600 species of bacteria in the human mouth. Over 75% of oral cancers are thought to be linked to smoking, but it remains unclear whether microbial differences in the mouth affect the risk for cancer.
Higher levels of Streptococcus in smokers' mouths
Researchers from New York University Langone Medical Center and its Laura and Isaac Perlmutter Cancer Center have been using precise genetic tests to investigate the impact of smoking on the composition and action of oral microbiota.
Fast facts about quitting smoking
- In the US, there are more former smokers than current smokers, according to the CDC
- In 2010, 68.8% of American smokers wanted to quit
- In 2013, 48% of smokers in high school had tried to quit in the past year.
The team used mouthwash samples from 1,204 American adults who are registered in a large, ongoing study into cancer risk, funded by the National Institutes of Health (NIH) and the American Cancer Society (ACS).
Participants were all aged 50 years or over. Among them were 112 smokers and 521 individuals with no history of smoking. There were also 571 people who had quit smoking, 17% of them having stopped within the past 10 years.
Using genetic tests and statistical data, the researchers analyzed the thousands of bacteria residing in the mouths of volunteers.
Results suggest that the oral microbiome of smokers is significantly different from that of people who have never smoked or are no longer smoking. In the mouths of smokers, the levels of 150 bacterial species were significantly higher, while levels of 70 other species were distinctly lower.
Proteobacteria made up 4.6% of overall bacteria in the mouths of smokers, compared with 11.7% in nonsmokers. Proteobacteria are thought to play a part in breaking down the toxic chemicals introduced by smoking.
By contrast, 10% more species of Streptococcus were found in the mouths of smokers, compared with nonsmokers. Streptococcus is known to promote tooth decay.
Recovery comes after quitting smoking
On quitting smoking, however, the oral microbiome appears to return to its previous state. In people who had smoked previously, but not in the last 10 years, the microbial balance was the same as in the mouths of nonsmokers.
Senior investigator and epidemiologist Jiyoung Ahn, PhD, says:
"Further experiments will be needed, however, to prove that these changes weaken the body's defenses against cancer-causing chemicals in tobacco smoke, or trigger other diseases in the mouth, lungs or gut."
Co-lead investigator Brandilyn Peters, PhD, points out that the results do not reveal how long it takes former smokers' microbiome to find its balance after quitting.
The authors are planning further studies to establish the precise timeline for recovery of the bacterial community in the mouth.
They also hope to understand the biological changes that occur in the oral microbiome as a result of smoking, and how these changes might affect the risk for various cancers of the mouth and elsewhere in the body.
Medical News Today recently reported on research suggesting that the effect of some smoking cessation therapies may be limited.