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Opportunistic Bacteria Are More Abundant in the Microbiome of Mouthwash Users

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Alcohol-based mouthwash may be having a detrimental effect on the oral microbiome, according to a new study.

In a follow-up to a trial on the use of mouthwash as a method to reduce the transmission of sexually transmitted diseases (STDs), researchers from the Institute of Tropical Medicine in Antwerp, Belgium, found that two species of opportunistic bacteria were significantly more abundant in the mouth after three months of daily use of an alcohol-based mouthwash.

Before individuals are encouraged to forgo mouthwash, however, the researchers urge that the study results do not mean that the general public should no longer use mouthwash.

The findings were published in the Journal of Medical Microbiology.

The oral microbiome: A window to overall health

The oral microbiome refers to the community of organisms, primarily bacteria, that exists within the oral cavity. These organisms help digest food and keep the mouth healthy. However, changes in the oral microbiome can induce diseases such as periodontal disease. The development of some cancers and various systemic diseases, including Alzheimer’s disease, have also been linked to alterations in the oral microbiome.

The current microbiome sub-study is a follow-up to an investigation on the use of mouthwash to reduce transmission of STDs among men who have sex with men. Fifty-nine men taking HIV pre-exposure prophylaxis (PrEP) were enrolled in the study where they received threemonths of daily mouthwash followed by threemonths of placebo mouthwash or vice versa. Oropharyngeal swabs were taken at the start of the study and after threemonths of use of each mouthwash.

“Alcohol-based mouthwashes are widely available. The public may use them daily to tackle bad breath or prevent periodontitis, but they should be aware of the potential implications. Ideally, long-term usage should be guided by healthcare professionals,” said Dr. Jolein Laumen, first author of the study and researcher at the Institute of Tropical Medicine.

Two species of bacteria were significantly more abundant in the mouth after three months of daily use of the alcohol-based mouthwash, Fusobacterium nucleatum and Streptococcus anginosus. These two bacteria have been previously linked to gum disease, oesophageal and colorectal cancers.

Researchers also saw a decrease in a group of bacteria called Actinobacteria which are crucial contributors to the regulation of blood pressure.

“Up to half of men who have sex with men report using mouthwash for oral hygiene. This study adds to the literature that this is likely having an adverse effect on their oral microbiomes,” said Professor Chris Kenyon, head of the Unit of Sexually Transmitted Infections at the Institute of Tropical Medicine.

These findings suggest that the regular use of mouthwash should be carefully considered, however, the researchers are reluctant to draw substantial conclusions from the data.

Information on dietary habits and smoking was not collected and, therefore, not accounted for in the analysis. In addition, as the small sample group was taken from the initial study, the results may not be representative of the entire population.

Further research should examine how the detected changes affect clinical outcomes in larger and more representative sample groups, the researchers conclude.

Reference: Laumen JGE, Van Dijck C, Manoharan-Basil SS, et al. The effect of daily usage of Listerine Cool Mint mouthwash on the oropharyngeal microbiome: a substudy of the PReGo trial. J Med Microbiol. 2024;73(6):001830. doi: 10.1099/jmm.0.001830

This article is a rework of a press release. Material has been edited for length and content.