This article was written by Jenny To, PhD.
Key points
- The oral cavity is more than just an entry point to the digestive system. A growing body of evidence continues to highlight this connection – especially in respect to the microbiomes and their impact on health.
- Signs of gastrointestinal (GI) issues – like IBD – can even be spotted during your regular dental checkup.
- Moreover, oral bacteria can travel directly to the gut by being swallowed. The extent to which they can multiply and cause trouble in the gut is determined by immune responses and barriers in the digestive system.
Below we’ll describe relationships between the oral microbiome, oral health, and digestive diseases.
How the oral and gut microbiomes compare
Although disease changes many aspects of the microbiome, here is a brief overview of how the oral microbiome and gut microbiome generally compare.
Who is there?
The oral microbiome is second only to the gut microbiome in terms of numbers of bacterial species – about 700 species in the oral cavity compared to 500 – 1000 species in the intestinal system. Alongside bacteria, both environments also house an undetermined amount of viruses, fungi, and protozoa. And there are even more species yet to be identified – known as microbial dark matter.
Key bacteria found uniquely in each environment:
In the mouth
- Streptococcus
- Gemella
- Fusobacterium
- Porphyromonas
- Prevotella
- Veillonella
- Actinomyces
- Neisseria
In the gut
- Bacteroides
- Lactobacillus
- Bacillus
- Clostridium
- Bifidobacterium
- Enterococcus
- Proteobacteria (like E. coli)
What are the conditions like?
In the mouth
The oral cavity has many different habitats including gum tissue and plaque below and above the gumline. Along tooth surfaces, oral bacteria assemble highly organized and stable biofilms in the form of dental plaque. Bacteria are also found on the tongue, tonsils, throat, buccal mucosa, hard palate, and importantly – saliva.
Many factors shape these microenvironments in the mouth, including pH, salt level, redox potential, oxygen, and available nutrients. As you’re well aware, dental hygiene also plays a huge role in shaping the oral microbiome.
In the gut
The conditions in the small and large intestines are distinct. In the small intestine, food moves quickly through, is broken down by acids, and its nutrients absorbed. The bacteria in the large intestine are largely comprised of anaerobes, which utilize food that wasn’t digested in the small intestine. Unlike the oral microbiome, the gut microbiome is less stable and more subject to shaping by host and environmental factors. These selective pressures help to maintain homeostasis by forming barriers – described below – between the immune system and microbial infection.
Do oral bacteria colonize the gut?
Not easily in a healthy individual. A comparison of saliva and stool samples taken from the same healthy subjects revealed that the bacterial composition of each sample were very different and distinct. Several other studies also report very little overlap in saliva and stool microbial composition. These bacteria have evolved with humans over time to survive in different conditions in the mouth and gut. If introduced to the gut microbiome, oral bacteria would have to compete for resources and space with native commensal (friendly) bacteria in the gut.
There are several barriers that prevent oral bacteria from setting up shop in a healthy digestive system. These include:
- Physical – like distance and intact mucus and epithelial layers
- Chemical – like gastric acid and bile
- Biochemical – like enzymes and antimicrobial proteins
- Immunological – like antibodies (IgA) and immune cells
