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In a recent study published in the journal Scientific Reports, researchers examined the changes in oral microbiota composition due to smoking.

Smoking is a risk factor for cardiovascular diseases, oropharyngeal cancers, periodontitis, and respiratory diseases, among others. Oral microbiota alterations have been reported in smokers and cases of periodontitis, cardiovascular diseases, celecoxib cardiac risk and squamous cell carcinoma. Thus, smoking-related oral microbiota changes may contribute to chronic conditions. Oral microbiota has several functions and is involved in the nitrate reduction pathway.

Study: Smoking and salivary microbiota: a cross-sectional analysis of an Italian alpine population. Image Credit: Lightspring / Shutterstock

Nitrate-rich diets elevate the abundance of nitrate-reducing bacteria (NRB). Whether tobacco modifies the relative abundance of NRB remains unclear. The composition of the salivary microbiota varies by smoking behavior. A meta-analysis noted that smokers had increased Actinobacteria, lower Proteobacteria, and reduced proportion of aerobes compared to never or former smokers.

Although associations between salivary microbiota composition and smoking status have been characterized in American populations, no study has examined the associations between salivary microbiota and metabolic potential with years since smoking cessation or smoking intensity in Europeans.

The study and findings

The present study investigated associations between salivary microbiota composition and smoking status, history, and intensity in an Italian cohort of 1,601 individuals. Participants were aged 45, on average, with nearly 53% of the cohort being female. Most subjects (72%) had ≥ 20 natural teeth, and 45% were former or current smokers. The primary tobacco source was cigarettes for all except five individuals.

Former and current smokers aged 41-60 with a higher cumulative smoking exposure had fewer teeth than those with lower exposure. DNA sequencing of oral microbiota from select samples yielded nearly 36 million reads, with 22,308 median reads per sample. The dataset contained 627 amplicon sequence variants from 82 genera. Firmicutes and Prevotella were the most dominant at the phylum and genus levels, respectively, in the cohort.

The microbiota composition was associated with smoking, age group, sex, and number of teeth. There was no significant association between alpha diversity and smoking status. Former and never smokers had highly similar microbiota composition. Forty-four genera significantly differed between never and current smokers, adjusted for the number of teeth, sex, and age.

Further, the relative abundance of aerobes was consistently reduced in smokers in favor of anaerobes. Next, the team found a positive association between Fretibacterium and increases in daily smoking intensity (grams of tobacco per day). Moreover, nine genera were differentially abundant between current and never smokers.

The average relative abundance of aerobic taxa significantly reduced with increasing smoking intensity, adjusted for number of teeth, age, and sex, whereas that of anaerobes increased slightly. Besides, the researchers explored associations between the microbiota of former smokers and years since quitting.

The microbiota profile of participants who quit smoking ≥ five years ago was more likely to resemble that of never smokers. The relative abundance of aerobic taxa showed a mild increase in the 20 years since quitting smoking. Finally, there were 21 differentially abundant pathways between never and current smokers.

Conclusions

To summarize, the study explored associations of salivary microbiota composition with smoking status, intensity, and years since quitting in an Italian cohort. Aerobes varied by the intensity and frequency of smoking exposure. Further, the salivary microbiota composition was generally similar between former and never smokers, particularly among those who quit smoking more than five years ago.

Multiple aerobic microbial pathways were reduced in smokers, including the ubiquinol synthesis and nitrate reduction pathways. Additionally, pathways requiring oxygen or those generating an excess reducing power were reduced, suggesting that smoking results in a hypoxic environment in the oral cavity. The lower abundance nitrate reduction pathway in smokers may additionally explain the effect of smoking on the risk of periodontal and cardiovascular diseases, which should be tested in future studies.

The study’s limitations are the cross-sectional design and the lack of assessment of gum health or the number of missing, filled, or decayed teeth. Moreover, residual confounding was possible despite controlling for the number of teeth, sex, and age due to alcohol intake, diet, and medication usage. Taken together, the study revealed an association between salivary microbiota changes and smoking. Aerobic taxa were the most sensitive to exposure to smoke, regardless of the phylogeny. Individuals who quit smoking five years ago had microbiota profiles similar to those of never-smokers.

Written by

Tarun Sai Lomte

Tarun is a writer based in Hyderabad, India. He has a Master’s degree in Biotechnology from the University of Hyderabad and is enthusiastic about scientific research. He enjoys reading research papers and literature reviews and is passionate about writing.