Causal relationship between gut microbiota and dental caries: a two-sample mendelian randomisation study
Emerging research has begun to explore the ‘gut-oral microbiota axis,’ suggesting that the trillions of microorganisms residing in the digestive tract may influence health outside the intestines.
Dental caries remains one of the most prevalent chronic diseases globally, traditionally attributed to oral bacteria such as Streptococcus mutans and dietary habits. However, emerging research has begun to explore the ‘gut-oral microbiota axis,’ suggesting that the trillions of microorganisms residing in the human digestive tract may influence health outcomes far beyond the intestines.
While previous studies established that oral dysbiosis (an imbalance of oral bacteria) could lead to gut issues, this study sought to investigate the reverse: whether the composition of the gut microbiota plays a causal role in the development and progression of dental caries. By understanding this relationship, researchers hope to identify new systemic pathways
for preventing and treating tooth decay.
To determine causality rather than simple correlation, we employed a Two-Sample Mendelian Randomisation (MR) analysis. This is a high-level statistical method that uses genetic variants (specifically Single Nucleotide Polymorphisms, or SNPs) as ‘instruments’ to represent environmental exposures, in this casem the presence of specific gut bacteria.
We utilised data from large-scale Genome-Wide Association Studies (GWAS):
- Exposure Data: Genetic information regarding gut microbiota composition from the MiBioGen consortium.
- Outcome Data: Genetic data related to dental caries.
By using genetic proxies, we could bypass the ‘confounding factors’ (like sugar intake or socioeconomic status) that often plague traditional observational studies, allowing us to isolate the specific biological effect of the gut microbiome on dental health.
Key findings
The study identified several specific bacterial groups in the gut that significantly influenced the risk of developing dental caries. These were categorised into those that increased risk (positive correlation) and those that appeared to offer protection (negative correlation).
Bacteria associated with increased risk:
Eubacterium brachy group: Found to be positively correlated with the incidence of dental caries.
Terrisporobacter: This genus also showed a positive correlation, suggesting that higher concentrations of these bacteria in the gut may predispose individuals to more frequent or severe tooth decay.
Bacteria associated with decreased risk (Protective):
- Escherichia/Shigella: Surprisingly, these showed a negative correlation, meaning their presence
was linked to a lower risk of caries. - Oscillibacter and Oscillospira: Both genera were identified as protective factors.
- Ruminococcaceae UCG014: Higher concentrations of this group were associated with a reduced likelihood of dental caries.
The study suggests that the gut microbiome influences the mouth through several systemic pathways:
- Inflammatory Response: Certain gut bacteria can trigger or suppress systemic inflammation. Since dental caries involves an inflammatory component in the dental pulp and surrounding tissues, a gut-driven systemic immune response may alter how the body defends against decay-causing oral bacteria.
- Metabolic Signaling: Gut bacteria produce metabolites (such as short-chain fatty acids) that enter the bloodstream and can influence the pH levels and mineral content of saliva, which are critical factors in the demineralization and remineralization of tooth enamel.
- The Second Brain: The researchers refer to the gut microbiome as the “second brain” due to its role in regulating the host’s overall health, suggesting that a healthy gut creates a systemic environment that makes the oral cavity less hospitable to pathogenic bacteria.
Clinical implications and future directions
The confirmation of a causal link between gut health and dental health has significant implications for both general medicine and dentistry:
- Probiotic Interventions: The findings suggest that specific probiotics designed to bolster “protective” gut bacteria (like Oscillibacter) could potentially be used as a supplementary treatment to prevent cavities in high-risk patients.
- Holistic Diagnostics: Dentists may eventually look toward gut health markers to predict a patient’s susceptibility to caries, moving beyond traditional dietary advice.
- Personalised Medicine: Understanding a patient’s genetic predisposition to certain gut microbiota could allow for personalised preventative strategies.
Conclusion
The study concludes that the gut microbiota is not merely a passive observer in oral health but an active participant. By identifying specific bacterial genera that influence the development of dental caries, this research provides a foundation for the “gut-oral microbiota axis” theory. It shifts the focus of dental health from a localized perspective (just the mouth) to a systemic one, highlighting that the secret to a healthy smile may partially reside in the health of the digestive system.
While further clinical trials are needed to see how dietary or probiotic changes to the gut directly impact the mouth, this Mendelian Randomisation study provides the strongest evidence to date that our internal microbial balance is a key determinant of dental longevity.