The oral microbiome

The advent of the Human Microbiome Project in 2007 ushered in a new era of human health research. This seminal National Institutes of Health initiative revealed the hidden microbial communities residing within the nasal and oral cavities, skin, and gastrointestinal and urogenital tracts. As a direct result of this massive collaborative effort, a subsequent wave of research studies has begun to explore the potential microbiome connections to many different human health conditions. This effort continues to the present day, and the results have been nothing short of groundbreaking. The key takeaway message is that despite their small size, cells from our microbiota play outsized roles in human health. In the 19th century, it was both shocking and controversial when Koch^1,2 suggested that the common plagues of humanity could be attributed to simple single-celled microscopic organisms. Today, we take this fact for granted. Now, more than a century after Koch’s discoveries, it seems we have come full circle with the human microbiome. What was once considered somewhat mundane microbial passengers in the human body are now understood to be critical for the proper functioning of a vast array of complex human biological processes. When these abilities go awry, there are significant consequences for human health.

Most of the attention given to the human microbiome has thus far centered on the gut microbiome because its communities comprise most of the total microbial biomass in the body as well as much of its biodiversity. Consequently, the oral microbiome has been largely overshadowed. However, this is quickly changing. Even though the oral cavity harbors only a small fraction of the microbial biomass found in the gut, total microbial biodiversity is actually not much lower than in the gut. The oral cavities of humans harbor nearly 800 different bacterial species along with a broad diversity of archaea, eukaryotes, and viruses.³ We have known for decades that the composition of these organisms is the key to understanding common oral diseases like caries and periodontitis. However, this is only a small piece of a much larger story because it is now understood that what happens in the mouth does not necessarily stay in the mouth. Oral microbes routinely translocate to the gut and lungs. The oral microbiota and its metabolites are also constantly injected into the bloodstream from everyday activities like eating and oral hygiene, whereas chronic oral inflammation from diseases like periodontitis introduces a wide array of immunomodulators into the circulation, affecting distant tissues.⁴ Similarly, oral microbiome research has revealed compelling associations with a surprisingly wide array of extraoral pathologies, such as multiple types of cancer,⁵ cardiovascular diseases,⁶ neurodegenerative diseases,^7,8 rheumatoid arthritis,⁹ adverse pregnancy outcomes,¹⁰ and many others.^11,12 The mechanisms supporting these associations are an emerging topic of tremendous interest in the field.

So, what does this mean for dentistry? First, there should be a common recognition of the oral cavity as a key component of systemic health. This begins with dental education for students and continuing education courses for clinicians. Public outreach is also critical. Preventative oral health care should be perceived as a simple, cost-effective strategy to improve overall health. Perhaps this would also nudge more insurers to include dental coverage as a standard component. In purely economic terms, the return on investment for prevention is a boon for insurers, especially when considering the exorbitant costs incurred with many of the aforementioned extraoral conditions and diseases. Second, our improved understanding of the oral microbiome presents exciting new opportunities to expand the services delivered by oral health care providers. Regular dental visits are ideally suited for the delivery of a wide range of disease risk assessments beyond caries and periodontitis. Oral health care providers could also play a direct role in surveilling and managing patient microbiome composition. A new dentistry paradigm could be (and should be) preventative systemic health care.

Disclosures

Drs Merritt and Ferracane did not report any disclosures.

Disclaimer

Dr Merritt serves as an Editorial Board member for JADA-FS and Dr Ferracane serves as the Editor-in-Chief for JADA-FS. Drs Merritt and Ferracane were not involved in decisions about the article they wrote, and peer review was handled independently.