Your mouth has its own unique microbiome – it’s the second largest one in the human body and one of the most well-studied. Dentists play a crucial role in our immune health; a regular dental checkup can improve the health of the oral cavity.1

The human mouth interacts with much of our everyday environment; beyond the foods and drinks we consume and the air we breathe, our mouths contact foreign objects, body parts, and – hopefully – a toothbrush a couple of times a day. These interactions drive the variable differences in the respective microbiomes of individuals and play an important role alongside the core microbiome species we see under healthy conditions.

Like other microbiomes, the oral microbiome is also composed of an ecosystem of bacteria, viruses, fungi, and yeast. But different from other microbiomes, the oral cavity has two types of surfaces that bacteria colonize: the hard and soft tissues of the teeth and the oral mucosa, respectively, which allows for preferential colonization by microorganisms due to their ability to bind to these different surfaces. 

The healthy oral microbiome

The oral cavity creates an ideal environment for the growth of microorganisms; therefore, it is vital to maintain its health with regular dental and medical checkups and daily cleaning. For starters, the mouth maintains a relatively constant 98.6° F (37° C) temperature, which is a comfortable temperature for both good and bad bacteria to survive. Next, saliva maintains a neutral pH, fluctuating between 6.5 and 7.5, which is favorable for many species of bacteria. Third, the moisture in the mouth hydrates the microorganisms and allows for easy transport of nutrients to them, facilitating their ability to thrive and disperse outside the mouth and into the rest of the body.2  

Despite ideal environmental conditions for bacteria to survive in the mouth, the oral microbiome can have large and rapid changes in its ecosystem. A few factors that play a role in these rapid changes include all the aspects of a meal – how much food, how often, what types, the oral cavity response to changes in pH, bacteria-to-bacteria interactions, and, over time, gene mutations or gene transfers that display new properties to the bacteria strains.3

There are two types of bacteria: aerobes that require oxygen and anaerobes that don’t. Together, they are essential for the balance of the microbial ecosystem. They form multi-species communities known as biofilms, which resist changes in their environment, helping the oral cavity maintain homeostasis.2 

Adults produce more than a liter of saliva daily, almost all of which enters the GI tract. Therefore, it is not surprising that many of the same core bacteria (both healthy and opportunistic) seen in the gut are also found in the mouth:

  • Abiotrophia
  • Peptostreptococcus
  • Streptococcus
  • Stomatococcus
  • Actinomyces
  • Bifidobacterium
  • Corynebacterium
  • Eubacterium
  • Lactobacillus
  • Propionibacterium
  • Pseudoramibacter
  • Rothia
  • Moraxella
  • Neisseria
  • Veillonella
  • Campylobacter
  • Capnocytophaga
  • Desulfobacter
  • Desulfovibrio
  • Eikenella
  • Fusobacterium
  • Hemophilus
  • Leptotrichia
  • Prevotella
  • Selemonas
  • Simonsiella
  • Treponema
  • Wolinella.

Read more about how in-home companion animals and exercise influence the microbiome ecosystem.

The oral microbiome and disease connection

It has been long known that there is a relationship between oral health and systemic conditions, most notably cardiovascular disease, adverse pregnancy outcomes, diabetes mellitus, and respiratory diseases.4 Emerging research is focusing on the association of oral dysbiosis, or an imbalance in the microorganism ecosystem, with other conditions, including systemic inflammation, cancer and tumor development, and Alzheimer’s disease.5

Periodontal disease is a risk factor for a variety of systemic diseases, and the research points to certain specific species. Porphyromonas gingivalis and Fusobacterium nucleatum are two primary periodontal disease pathogens that cause inflammation and destruction of the intestinal barrier, induction of endotoxemia, and a systemic inflammatory response as the microorganisms travel from the mouth into the bloodstream or down the GI tract.6  

One bug, Streptococcus salivarius, is an early colonizer in the oral cavity. It also has the capability to live further down the intestinal tract and is known to down-regulate nuclear factor-kappaB (NF-kB) in small intestinal epithelial cells, decreasing inflammation and positively impacting systemic homeostasis.7

In individuals who have inflammatory bowel disease (IBD), research has found differences in oral bacteria compared to healthy individuals. Streptococcus, Prevotella, Neisseria, Haemophilus, and Veillonella are the largest oral bacterial players. Interestingly, F. nucleatum is also one of the main oral bacteria that are seen in tissue samples down the GI tract in individuals with IBD. In addition, Klebsiella pneumoniae from the oral cavity can enter the GI tract and can increase T-helper 1 white blood cells in the intestinal mucosa, which can induce IBD.8 The research finds that more oral-derived microorganisms can be detected in the intestinal mucosa and feces of patients with IBD than in the intestines of healthy individuals.

The oral flora of individuals with liver cancer tends to have more Bacillus, Leptotrichia, Actinomyces, and Campylobacter, and lower abundances of Haemophilus, Streptococcus, and Pseudomonas compared to healthy individuals. Many clinical studies show a relationship between P. gingivalis and periodontitis with non-alcoholic fatty liver disease and its progression.5 And the role of P. gingivalis in increasing intestinal permeability can result in an increased risk for insulin resistance, the spread of intestinal bacteria to the liver, and increased triglyceride content in liver tissue.5

Endocarditis, which is inflammation of the lining of the heart chambers and values, also has a link back to oral health. Lactobacillus and Streptococcus in the oral cavity are related to the occurrence of infective endocarditis. Specifically, Streptococcus sanguis is an early colonizer of dental plaque and is closely related to the inflammatory process of endocarditis with the highest detection rate in the endocardium of endocarditis patients.9

The connection between oral health and Alzheimer’s disease appears to be multifactorial. Multiple studies in the past decade have found many oral factors are associated with Alzheimer’s, including severity of periodontitis, number of missing teeth, tooth brushing habits, bleeding gums, and bone loss in the mouth. In mice models, these variables have a connection back to the oral flora and how well the body is able to decrease inflammation and maintain homeostasis.5

Although we still are only scraping the surface of the oral microbiome knowledge, the relationship between the structure and function of oral microbes and the balance of human health and disease is becoming clearer but more complex. A public database called the Human Oral Microbiome Database serves as a repository for oral bacterial genome sequences and an in-depth resource consisting of oral bacterial taxa and identification. This database will continue to help researchers profile our body’s various microbiomes for a better understanding of the tiny microorganisms we interact with on a daily basis that play an immense role in our health and wellness. 

References

  1. Deo PN, Deshmukh R. Oral microbiome: Unveiling the fundamentals. J Oral Maxillofac Pathol 2019;23(1):122-128.
  2. Lim Y, Totsika M, Morrison M, Punyadeera C. Oral microbiome: A new biomarker reservoir for oral and oropharyngeal cancers. Theranostics 2017;7(17):4313-4321. McLean JS. Advancements toward a systems level understanding of the human oral microbiome. Front Cell Infect Microbiol 2014;4:98.
  3. Weidlich P, Cimões R, Pannuti CM, Oppermann RV. Association between periodontal diseases and systemic diseases. Braz Oral Res 2008;22 Suppl 1:32-43.
  4. Peng X, Cheng L, You Y, et al. Oral microbiota in human systematic diseases. Int J Oral Sci 2022;14(1):14.
  5. Kato T, Yamazaki K, Nakajima M, et al. Oral administration of Porphyromonas gingivalis alters the gut microbiome and serum metabolome. mSphere 2018;3(5). doi:10.1128/mSphere.00460-18
  6. Li Y, Shao F, Zheng S, Tan Z, He Y. Alteration of Streptococcus salivarius in buccal mucosa of oral lichen planus and controlled clinical trial in OLP treatment. Probiotics Antimicrob Proteins 2020;12(4):1340-1348.
  7. Kitamoto S, Nagao-Kitamoto H, Jiao Y, et al. The intermucosal connection between the mouth and gut in commensal pathobiont-driven colitis. Cell 2020;182(2):447-462.e14.
  8. Martini AM, Moricz BS, Ripperger AK, et al. Association of novel Streptococcus sanguinis virulence factors with pathogenesis in a native valve infective endocarditis model. Front Microbiol 2020;11:10.