The effect of daily usage of Listerine Cool Mint mouthwash on the oropharyngeal microbiome

Listerine is a bactericidal mouthwash used to prevent oral health problems such as dental plaque and gingivitis. It is unknown whether it promotes or undermines a healthy oral microbiome. We aimed to assess if daily usage of Listerine Cool Mint® influenced the composition of the pharyngeal microbiome. The current microbiome substudy is part of the PreGo trial conducted in 2019-2020, wherein sixty-four MSM taking HIV PrEP were enrolled. In this crossover trial, participants were received 3 months of daily Listerine followed by 3 months of placebo mouthwash or vice versa. Oropharyngeal swabs were taken at baseline and after 3 months use of each mouthwash. DNA was extracted for shotgun metagenomic sequencing (Illumina Inc.,). Non-host reads were taxonomically classified with MiniKraken and Bracken. The diversity indices alpha and beta diversity were compared between baseline and after each mouthwash use. Differentially abundant bacterial taxa were identified using ANOVA-Like Differential Expression analysis. Streptococcus was the most abundant genus in most samples (n=103, 61.7%) with a median relative abundance of 31.5% [IQR 20.6 to 44.8], followed by Prevotella (13.5%, [IQR 4.8 to 22.6]) and Veillonella (10.0%, [IQR 4.0 to 16.8]). Compared to baseline, the composition of the oral microbiome at the genus level (beta diversity) was significantly different after 3 months Listerine (P=0.006, pseudo-F = 2.29) or placebo (P=0.003, pseudo-F = 2.49, PERMANOVA) use. Fusobacterium nucleatum and Streptococcus anginosus were significantly more abundant after Listerine use compared to baseline. Listerine use was associated with an increased abundance of common oral opportunistic bacteria previously reported to be enriched in periodontal diseases, oesophageal and colorectal cancer, and systemic diseases. These findings suggest that the applications of Listerine mouthwash should be carefully considered.