Nobody still thinks smoking is good for you, but we can now add another item to the towering pile of reasons to quit: smoking can throw off the balance of helpful and harmful bacteria living in your mouth. These findings were published last week in The ISME Journal.

The microorganisms colonizing your body are more important—and more influential—than most people realize. There are bacteria, viruses, and even fungi living all over your body, inside and out. The genetic makeup of a microbial community is called a microbiome. And the health, diversity, and balance in each microbiome can mean the difference between sickness and health.

A healthy mouth is home to about 600 different species of bacteria. Scientists know that those bacteria are sensitive to what we eat and how we care for our teeth, but few studies have examined how they might be affected by smoking. So researchers at New York University started collecting spit.

They recruited 1204 people who had already enrolled in separate cancer risk studies at the National Institutes of Health and the American Cancer Society. Of those people, 112 were smokers, 571 former smokers, and 521 nonsmokers. Each study participant was given about two teaspoons of Scope mouthwash, which they swished around in their mouth and then spit into a test tube. The spit was frozen and sent for DNA analysis to sort out the bacterial contents of each participant’s oral microbiome.

As expected, smoking was not a great influence on oral bacteria. The overall profile of their microbiota looked very different from that of nonsmokers. It wasn’t just that they had less of some species; they also had more of others, including 10 percent more Streptococcus species—that’s strep bacteria—than nonsmokers. The researchers found that smokers had higher levels of 150 different bacterial species and lower levels of another 70.

There is some good news in all this. The mouths of former smokers, especially those who had quit 10 or more years ago, were identical in bacterial profile to the mouths of nonsmokers. Their oral microbiota were able to bounce back after they quit smoking, although the researchers weren’t able to determine exactly how long it took.

"Our study is the first to suggest that smoking has a profound impact on the oral microbiome," senior investigator and epidemiologist Jiyoung Ahn said in a press statement. "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."