The human mouth is a jungle of teeth, tonsils, gingiva, and tongue colonized by bacteria. Up to 700 bacterial species make up the oral microbiome: The good type helps you digest food and fight bad breath, and the bad causes oral diseases and some cancers. Some people — in particular heavy drinkers — have more of the bad stuff than others, report scientists in Microbiome on Tuesday.

This paper is the first to directly compare drinking levels and their effects on oral bacteria, and the findings aren’t great for people who enjoy their booze. Previous research had linked microbial changes in the mouth to an increased risk for head, neck, and gastrointestinal cancers, and the researchers behind the study wanted to better understand the connection as well as determine what might cause the microbial changes. Seeing as an estimated 10 percent of American adults are considered heavy drinkers — defined as one or more drinkers per day for women and two or more drinks per day for men — looking at alcohol consumption seemed like as good a place as any to start.

mouth bacteria
Bacteroidales was one of the potential harmful species of bacteria found in the mouth of drinkers.

“Our study offers clear evidence that drinking is bad for maintaining a healthy balance of microbes in the mouth and could help explain why drinking, like smoking, leads to bacterial changes already tied to cancer and chronic disease,” senior investigator and NYU Langone Health epidemiologist Jiyoung Ahn, Ph.D. explained in a statement released Monday.

Ahn and her team examined the oral bacteria of 1,044 predominantly white, healthy people between the ages of 55 and 87. This group was divided into 270 nondrinkers, 614 moderate drinkers, and 160 heavy drinkers. After genetically sorting and quantifying the samples, the scientists determined that drinkers had high levels of Bacteroidales, Actinomyces, and Neisseria, which are species of bacteria considered potentially harmful. The drinkers also had fewer Lactobacillales, which are lactic acid bacteria that promote the reduction of gum inflammation.

Ahn and her team hypothesize that the acids in alcoholic beverages might be turning the “oral environment hostile” for more healthy bacteria. It’s not clear how these bacterias cause disease, but the fact that Neisseria can synthesize acetaldehyde from ethanol is worth considering: The compound is thought to be a human carcinogen and is linked to most of the negative clinical effects of alcohol. Now that the scientists are aware of the bacterial imbalance in drinkers’ mouths, however, they can start looking for ways to rebalance it for prevention’s sake.

The next steps for this team include determining exactly what biological mechanisms are behind alcohol’s effect on mouth bacteria, and seeing whether all types of alcohol have the same effects, or whether our wine, beer, or liquor habits are to blame for our off-balance bacteria.