Links Between Diseases and Bacteria in your Mouth


Breaking down the microbiology world one bite at a time

Links Between Diseases and Bacteria in your Mouth

Though microbes like bacteria inhabit the human body inherently, there are certain areas where their presence drastically increases. These areas include your respiratory and digestive tracts, both of which are associated with the mouth. It’s odd to imagine, but your very own mouth is home to billions of individual bacteria, consisting of around 700 different species. They comfortably reside between your teeth, on your tongue and gums, and anywhere else you can imagine. 

The mouth is an optimal environment for the establishment of a microbial community, and a big reason is that bacteria like to eat sugar. In fact, they like it so much that they convert the sugar you eat into an acidic substance that ultimately destroys your teeth. Dental caries or cavities are caused by these sugar-munching microbes, usually in combination with a sugary diet. In the past, cavities were considered the main concern when it comes to oral bacteria. However, new research is showing that the bacteria in your mouth are linked to much more serious diseases. 

Starting with a big one: cancer. Oral bacteria play a role in the development of all kinds of cancers, not just the ones affecting the mouth. Oral squamous cell carcinoma (OSCC) is a type of cancer that affects tissue in the oral cavity. Previously, the main risk factors were smoking, alcohol abuse, and HPV contraction. Recently, connections have been made between tumor formation and the increased presence of certain bacteria in the mouth, such as P. gingivalis, F. nucleatum, and T. denticola. P. gingivalis is the main culprit, as it possesses a whole slew of factors promoting tumor formation. This bacterium can block the cell’s “damage-detection” methods, thus leaving it unable to know if it is becoming a cancer cell. It is also able to wreak havoc on the oral environment as a whole, and not just individual cells. Tumors grow best in inflammatory conditions, which are often caused by the overproduction of certain proteins, called cytokines, by immune cells. P. gingivalis produces an inflammation-causing toxin called lipopolysaccharide (LPS), which binds to receptors on immune cells. This prompts immune cells to release massive amounts of cytokines, triggering an inflammatory environment in which tumors can form and proliferate. 

Figure 1: Creation of an inflammatory environment for tumor formation. Image source: Image created in BioRender by author.

When P. gingivalis is joined by other microbes, it promotes tumor growth more effectively. The tissue of OSCC patients is heavily colonized by the three aforementioned species, which have also been identified in those suffering from other kinds of cancer. These include esophageal, colorectal, and even pancreatic cancers. Cancer is a dreadful illness, but what about the world’s number one killer? 

Heart disease is a term that broadly describes several serious conditions affecting the heart and surrounding blood vessels. Commonly, heart disease refers to the buildup of plaque in the body’s arteries; which is known as atherosclerosis. Plaques form when certain materials, such as cholesterol, enter the bloodstream and lodge in the walls of arteries. White blood cells patrolling the blood recognize these materials and trap them. This is a highly inflammatory process in which smooth muscle cells lining arteries multiply, which continues to wall off the foreign materials and eventually forms a plaque. If blood pressure gets too high, the smooth muscle coat may rupture and release the plaque into the bloodstream as a clot, which is a common cause of heart attack or stroke. People who have suffered from these conditions are often told to change their diet and lifestyle. Though this is good advice, they may also be advised to visit a dentist. 

Periodontal disease, also known as gum disease or periodontitis, is the creation of an inflammatory environment caused by bacterial overgrowth in the mouth. This causes a weakening and breakdown of gum tissue, allowing bacteria to easily enter the bloodstream. Once this happens, blood vessels become a warzone of inflammation as white blood cells detect the invading bacteria. This triggers the smooth muscle cells comprising arteries to multiply and form plaques. These plaques are just as dangerous as those resulting from high cholesterol, once again proving the importance of oral hygiene. 

Both cancer and heart disease have many identified causes, and neither ailment is particularly mysterious to scientists. However, in the case of Alzheimer’s, no definitive cause has been determined. The disease is defined by the formation of plaques, known as amyloids, in the brain. These plaques cause devastating symptoms such as memory loss, personality changes, and confusion. 

Figure 2: MRI imaging showing differences between healthy control group brains and those with Alzheimer’s Disease, which have more black spaces (indicates degeneration of tissue). Image source: LifeSensors. 

Common explanations for this process include hereditary factors, mainly the mutation of a certain gene (APOE). Though having the mutation by chance or a family history of the disease may increase one’s risk of developing it, these are not considered definitive causes. Similarly, oral bacteria and periodontal diseases have very recently been identified as additional risk factors. Studies have shown that things like irregular tooth brushing, degree of periodontal disease, and tooth loss are linked to further cognitive decline associated with Alzheimer’s disease. 

In lab studies, mice with P. gingivalis-induced periodontal disease show symptoms of an Alzheimer’s-like disease, as well as the formation of amyloid plaques. The specific way bacteria like P. gingivalis cause plaque deposition is still not well understood, though there are various explanations as to how the bacteria can directly impact brain tissue. Due to the presence of a blood-brain barrier, harmful substances and pathogens are not able to enter brain tissue and cause damage. As people age, however, this barrier becomes much more permeable. Those with periodontal disease have a highly inflammatory environment in their mouths, meaning there is a high concentration of pro-inflammatory cytokines in the surrounding blood circulation. Because the blood-brain barrier of an aged person is more permeable, these potent cytokines are much freer to enter brain tissue and wreak havoc. Another explanation is that bacteria enter brain tissue through peripheral nerves, which do not involve the blood-brain barrier. 
The causative relationship between the oral microbiome and certain chronic diseases has not been fully proven. Considering this, several recent studies have provided a clear correlation between periodontitis and an increased risk of developing these illnesses. An inflammatory oral environment has several deteriorative effects on the whole body, and this article highlights the importance of maintaining good oral hygiene.

Link to the original post: Peng, X., Cheng, L., You, Y. et al. Oral microbiota in human systematic diseases. Int J Oral Sci 14, 14 (2022).

Featured image: Microbes on tooth – dental/stores