Introduction The impact of nicotine, the addictive component of both traditional cigarettes and e-cigarettes, on many physiological processes remains poorly understood. To date, there have been few investigations into the impact of nicotine on the gut microbiome, and these studies utilized oral administration rather than inhalation. This study aimed to establish if inhaled nicotine alters the gut microbiome and the effect of sex as a biological variable. Methods Female (n=8 air; n=10 nicotine) and male (n=10 air; n=10 nicotine) C57BL6/J mice were exposed to air (control) or nicotine vapor (12 hour/day) for 13 weeks. A fecal sample was collected from each mouse at the time of sacrifice, and the gut microbiome was analyzed by 16S rRNA gene sequencing. QIIME2, PICRUSt, and STAMP were used to detect gut bacterial differences and functional metabolic pathways. Results Sex-specific differences were observed in both alpha and beta diversities in the absence of nicotine. While nicotine alters microbial community structure in both male and female mice as revealed by the beta diversity metric, nicotine significantly reduced alpha diversity only in female mice. A total of 42 bacterial taxa from phylum to species were found to be significantly different among the treatment groups. Finally, analysis for functional genes revealed significant differences in twelve metabolic pathways in female mice and ten in male mice exposed to nicotine compared to air controls. Conclusions Nicotine inhalation alters the gut microbiome and reduces bacterial diversity in a sex-specific manner, which may contribute to the overall adverse health impact of nicotine. Implications The gut microbiota plays a fundamental role in the well-being of the host, and traditional cigarette smoking has been shown to affect the gut microbiome. The effects of nicotine alone, however, remain largely uncharacterized. Our study demonstrates that nicotine inhalation alters the gut microbiome in a sex-specific manner, which may contribute to the adverse health consequences of inhaled nicotine. This study points to the importance of more detailed investigations into the influence of inhaled nicotine on the gut microbiota.
Nicotine is the addictive substance found in cigarettes and e‐cigarettes; however, nicotine's effect on many physiological processes has not been clearly delineated. A limited number of studies have investigated the impact of nicotine on the gut microbiome and most studies used oral administration instead of inhalation. During the last decade, the gut microbiome has been shown to influence host homeostasis, and its disruption (dysbiosis) has been associated with disease pathogenesis. This study aimed to determine if inhaled nicotine alters the gut microbiome and if an animal's sex can influence those changes. In this study, female (n=8 air and n=10 nicotine) and male (n=10 air and n=10 nicotine) C57BL6/J mice were exposed to air (control) or nicotine vapor (daily, 12 hours on/12 hour off) for 13 weeks. Serum cotinine, a measure of nicotine intake, was comparable to that observed in human smokers; interestingly, male mice had significantly higher levels than female mice. Body weight was significantly lower following nicotine vapor exposure in both male and female mice. A fecal sample was collected from each mouse at the time of sacrifice, followed by DNA isolation and 16S rRNA gene sequencing analysis of gut microbes. Differences in alpha and beta diversity were analyzed using Shannon and Bray‐Curtis statistics, respectively. Alpha diversity measures the diversity within a community, while beta diversity measures the pairwise similarity of communities. Nicotine significantly reduced alpha diversity in female mice but not male mice. In contrast, nicotine significantly altered community structure in both male and female mice producing clearly distinct clusters based on the Bray‐Curtis beta diversity metric. In addition, sex‐specific differences were observed in both alpha and beta diversity with air inhalation. In conclusion, we found that nicotine inhalation reduces the gut microbiome's bacterial diversity in a sex‐specific manner. Reduced diversity of the gut microbiome may contribute to nicotine's negative health impact.
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