Chlorhexidine Mouthwash Could Make Saliva Significantly More Acidic

Dentistry Today


Chlorhexidine mouthwash significantly increases the abundance of lactate-producing bacteria that lower saliva pH and may increase the risk of tooth damage, according to the University of Plymouth.

In the study, which researchers call the first looking at the effect of chlorhexidine mouthwash on the entire oral microbiome, subjects used a placebo mouthwash for seven days, followed by seven days of a chlorhexidine mouthwash.

At the end of each week, the researchers analyzed the abundance and diversity of the bacteria in the mouth and measured pH, saliva buffering capacity (or the ability to neutralize acids in the mouth), lactate, glucose, nitrate, and nitrate concentrations.

The chlorhexidine mouthwash led to a greater abundance of species within the Firmicutesand Proteobacteriafamilies as well as fewer Bacteroidetes, TM7, and Fusobacteria. This change was associated with an increase in acidity, seen in lower salivary pH and buffering capacity.

Though chlorhexidine reduced microbial diversity in the mouth, the researchers noted that more research was needed to determine if this reduction in diversity itself increased the risk of oral disease.

Saliva maintains a neutral pH in the mouth as acidity levels fluctuate as a result of eating and drinking. If saliva pH gets too low, damage can occur to the teeth and mucosa.

The research also confirmed previous studies indicating that chlorhexidine disrupted the ability of oral bacteria to turn nitrate into nitrite, a key molecule for reducing blood pressure. Lower saliva and blood plasma nitrite concentrations were found after using chlorhexidine mouthwash, followed by a trend of increased systolic blood pressure.

These findings supported earlier research led by the university showing that the blood pressure-lowering effect of exercise is significantly reduced when people rinse their mouths with antibacterial mouthwash rather than water as well.

“There is surprising lack of knowledge and literature behind the use of these products,” said Raul Bescos, PhD, of the university’s Faculty of Health.

“Chlorhexidine mouthwash is widely used, but research has been limited to its effect on a small number of bacteria linked to particular oral diseases, and most has been carried out in vitro,” said Bescos.

“We believe this is the first study to look at the impact of seven-day use on the whole oral microbiome in human subjects,” Bescos said.

“We have significantly underestimated the complexity of the oral microbiome and the importance of oral bacteria in the past,” said Louise Belfield, PhD, coauthor and lecturer at the Peninsula Dental School at the University of Plymouth.

“Traditionally, the view has been that bacteria are bad and cause diseases. But we now know that the majority of bacteria, whether in the mouth or the gut, are essential for sustaining human health,” said Belfield.

“As dental clinicians, we need more information on how mouthwashes alter the balance of oral bacteria, so we can prescribe them correctly. This paper is an important first step in achieving this,” said Zoe Brookes, PhD, BDS, coauthor and lecturer at the Peninsula Dental School. 

“In the face of the recent COVID-19 outbreak, many dentists are now using chlorhexidine as a pre-rinse before doing dental procedures. We urgently need more information on how it works on viruses,” said Brookes.

The study, “Effects of Chlorhexidine Mouthwash on the Oral Microbiome,” was published by Scientific Reports.

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