If you use chlorhexidine mouthwashes in your daily hygiene routine, this interests you. And a lot. The reason is that a team of researchers at the University of Plymouth (UK) has found that these products significantly increase the amount of lactate-producing bacteria, which reduce the pH of saliva (acidifying), which is more likely to cause tooth damage.
Participants in the experiment were given a placebo mouthwash for seven days. They were then offered a mouthwash for another week, this time chlorhexidine. The researchers analyzed the abundance and diversity of bacteria in the mouth (the oral microbiome), measured the pH, the ability of saliva to neutralize acids in the mouth and the concentrations of lactate, glucose, nitrate and nitrite for each of the periods.
What they saw was that the use, for seven days, of mouthwash with chlorhexidine meant that there were more bacteria in the mouth from the families Firmicutes and Proteobacteria, and less Bactericides, TM7 and Fusobacteria. This change was associated with an increase in acidity, which was observed at a lower salivary pH and in the ability of saliva to neutralize acids in the mouth.
The purpose of saliva, amongst other things, is to maintain a neutral pH in the mouth. Acidity levels fluctuate as a result of eating and drinking. If the pH of the saliva is too low, damage may occur to the teeth and mucosa, the tissue surrounding the teeth and inside the mouth.
In general, the researchers observed that chlorhexidine, which is widely used, reduces the microbial diversity of the mouth. However, the researchers warn that more research is needed to determine that this reduction in itself increases the risk of oral disease.
This study, published recently in Scientific Reports, found that chlorhexidine alters the ability of oral bacteria to convert nitrate to nitrite (a key molecule for reducing blood pressure). Lower nitrite concentrations were found in saliva and blood plasma after use of chlorhexidine mouthwash, followed by a trend of increased systolic blood pressure. These findings support previous research led by the same university, which showed that the blood pressure lowering effect of exercise is significantly reduced when people rinse their mouths with such a mouthwash instead of water.
"There is a surprising lack of knowledge and literature behind the use of these products. Chlorhexidine mouthwash is widely used, but research has been limited to its effect on a small number of bacteria linked to certain oral diseases, and most have been carried out in vitro", commented Raúl Bescos, the director of the investigation.
“We have significantly underestimated the complexity of the oral microbiome and the importance of oral bacteria in the past. Traditionally, the opinion has been that bacteria are bad and cause disease. But we now know that most bacteria - either in the mouth or in the intestine - are essential to maintaining human health,” said Louise Belfield, co-author of the study.
“As dental clinicians, we need more information about how mouthwashes upset the balance of oral bacteria, so that we can properly prescribe them. This document is an important first step towards achieving this,” said co-author Zoe Brookes.
Experts say that, in the recent coronavirus pandemic, many dentists are using chlorhexidine as a pre-wash before doing a dental procedure, but more information is needed on how this substance works on viruses.
Reference: Bescos, R., Ashworth, A., Cutler, C. et al. Effects of Chlorhexidine mouthwash on the oral microbiome. Sci Rep 10, 5254 (2020). https://doi.org/10.1038/s41598-020-61912-4.