Can Mouth Odor Come from the Tongue Surface as Well as the Teeth?

Even when teeth are clean, mouth odor can still be a concern. A 2001 study examined compounds in the breath of healthy people and identified tongue coating and the bacteria living in it as important sources.

The central focus was a sulfur-containing compound called methyl mercaptan. However, the study did not start from the premise that this one compound alone explains oral malodor. Through chemical analysis, odor evaluation, cleaning, and bacterial culture, it examined which substances contribute to unpleasantness, where in the mouth they are produced, and what materials are used to make them.

Even healthy people’s breath contains several odor compounds

The participants were nonsmoking adult men without major oral or systemic disease. Small groups were studied in each experiment: three people for the main compound analysis, seven for the relationship between mouth air and unpleasantness, and repeated measurements in three people for the cleaning comparison.

For the compound analysis, approximately 180 liters of breath exhaled during normal breathing were collected per person, concentrated, and measured. This collection method was distinguished from that for mouth air sampled after the mouth had been kept closed. The analysis captured even substances present in small quantities and detected 15 compounds, including lower fatty acids, nitrogen-containing compounds, and sulfur-containing compounds.

Some substances, such as acetic acid, were abundant but did not reach the concentration at which their odor could be recognized. Others, including butyric acid and methyl mercaptan, exceeded their recognition thresholds even in small amounts. Here, a threshold is a guide to the concentration at which an odor can be recognized. Simply ranking compounds by the amount detected does not identify which ones contribute to unpleasantness.

More methyl mercaptan was associated with greater unpleasantness

Methyl mercaptan exceeded its recognition threshold in the exhaled breath of all three people analyzed, and its concentrations were higher in mouth air than in exhaled breath. It was also present in the gas above cultured saliva, so it was selected as an indicator for subsequent experiments as a leading candidate produced inside the mouth.

In a test in which people smelled diluted mouth air from seven participants, higher concentrations of this compound were significantly associated with greater unpleasantness at threefold and tenfold dilution. At thirtyfold dilution, the relationship pointed in the same direction but was not statistically clear.

Meanwhile, mouth air could smell more unpleasant than methyl mercaptan alone at the same concentration. The author discussed the contribution of multiple components, including fatty acids and nitrogen compounds, to the quality and unpleasantness of oral malodor. The results indicate that methyl mercaptan is a useful indicator, but that the entirety of oral malodor cannot be represented by one number.

Measurements following a small number of people from morning to night also showed a tendency for concentrations to be high on waking, decrease after meals, and rise again with time. Mouth odor can thus be understood as something that varies with time and conditions in the mouth rather than as a fixed characteristic of a person.

Tongue cleaning greatly reduced the capacity to produce the odor compound

To investigate the site of production, the researchers compared rinsing with water, brushing the teeth for three minutes without toothpaste, and cleaning the tongue surface with a dedicated instrument. Three people participated, with nine observations without cleaning, nine after water rinsing, five after tooth-surface cleaning, and six after tongue-dorsum cleaning. These are numbers of experimental observations obtained repeatedly from the same people, not numbers of participants.

A distinctive feature was that participants rinsed with L-methionine, a sulfur-containing amino acid, to stimulate production before measurement. Providing a substrate to increase production was intended to reveal differences that are small and difficult to measure in ordinary mouth air.

A production index based on the ratio of methyl mercaptan concentrations before and after cleaning was, in the comparison 30 minutes after cleaning, 44.3% lower with water rinsing, 71.8% lower with tooth-surface cleaning, and 88.8% lower with tongue-dorsum cleaning than with no cleaning. Tongue cleaning showed a larger reduction than the other cleaning procedures.

The figure of 88.8% describes a change in a production index in an experiment with added substrate. It does not mean that the unpleasantness of ordinary breath fell by the same percentage. Given its small sample and repeated measurements, the experiment’s role was to investigate whether the tongue surface was an important production site.

Culture experiments linking tongue coating, bacteria, and amino acids

Next, saliva, dental plaque, and tongue coating were collected from six people and cultured under oxygen-poor and oxygen-containing conditions. Under oxygen-poor conditions, the compound was produced even without added methionine, with particularly high production from tongue coating. Adding methionine increased production further, with the highest production in tongue coating, followed by plaque and saliva. Production was much lower in oxygen-containing cultures.

The activity of the bacteria themselves was also studied in 71 strains representing 42 species in 17 genera. Strains of Porphyromonas, Fusobacterium, and spirochetes, known for their association with periodontal disease, showed high methyl mercaptan production.

Another focus was Veillonella, a genus also present in healthy mouths. Some of the strains tested, particularly two, showed a large increase in production when methionine was added. One of these had been newly isolated from tongue coating. The author considered that not only disease-associated bacteria but also normal oral residents could contribute to odor production depending on the substrates available to them.

These findings are easier to understand when tongue coating is viewed not merely as an amount of debris, but as a place where bacteria live and break down substrates to produce volatile compounds. However, bacterial properties differ between strains, and high production in culture cannot be directly translated into a proportional contribution inside the mouth.

What can be learned from this study?

The author concluded that limiting the accumulation of plaque and tongue coating is important when considering odor production. The method of stimulating production with methionine was also proposed as a way to measure small changes in oral malodor and investigate its sources in healthy people.

However, this was a series of small studies in healthy adult men. It did not establish long-term cleaning effects or optimal tongue-cleaning force or frequency. Application to disease-associated oral malodor or to people of various ages and sexes requires separate investigation.

Summary

The mouth odors of healthy people contained sulfur compounds, fatty acids, and nitrogen compounds, among others. Methyl mercaptan, in particular, was related to unpleasantness and served as an indicator for studying odor production.

The cleaning and culture experiments support links between compound production and tongue coating, oxygen-poor conditions, bacteria, and the substrate methionine. Tongue cleaning produced a lower production index than water rinsing or tooth cleaning, and some Veillonella strains were also found to have high production capacity. The study indicates that understanding mouth odor requires attention not only to the teeth but also to the tongue surface and the microbial activity taking place there.

The author was affiliated with the Lion Foundation for Dental Health at the time of the research. The acknowledgments also note advice and cooperation on measurements and experiments from Lion Corporation’s Biological Science Center. No separate funding or conflict-of-interest declaration section was identified in the original paper.

Reference

Shibuya K. Constituents and Origins of Physiological Malodor. Journal of Dental Health. 2001;51(5):778–792. Original article (Open full text).