Gum Disease Opens Up the Body to a Host of Infections
By Laura Biel | Science News
For centuries, the mouth and the body have been disconnected — at least when it comes to health care. Through the Middle Ages and beyond, teeth fell under the care of barbers, who could shave a customer and pull a molar with equal skill. In the 1700s, French surgeon Pierre Fauchard published the Treatise on Teeth, establishing dentistry as its own science.
Across the channel in England, as physicians gained stature in the 19th century, surgeons and dentists engaged in a power struggle. In the modern United States, after medicine became linked to employer insurance and Medicare, the fissure between medicine and dentistry widened. Insurance coverage began at the throat.
So when Salomon Amar, a periodontal specialist at Boston University, began exploring links between oral bacteria and heart disease in animal studies in the late 1990s, reactions were lukewarm. “Many cardiologists thought we were a bit crazy,” he says. Skepticism still abounds, but the same molecular tools that have dramatically changed understanding of the gut microbiome are now allowing scientists to track and examine bacteria in the mouth. Advocates of a connection between the artery disease atherosclerosis and microbes are hoping to find convincing proof of their suspicions, while exploring links between ailing gums and other conditions, including cancer, arthritis, diabetes and even Alzheimer’s disease.
The work has profound implications for public health, given that more than 65 million American adults are thought to have periodontal disease, which occurs when bacterial overgrowth inflames the gums and can lead to erosion of gums and bone. If it turns out that periodontal decay drives other diseases, doctors would have a new, and relatively simple, means of prevention.
Wenche Borgnakke, a dental researcher at the University of Michigan in Ann Arbor, has been making this case for years, citing “solid evidence that periodontal treatment has an effect on systemic disease.” Shepoints to a study published last year in the journal Medicine comparing patients on dialysis who received periodontal treatment with those who did not. Those getting treatment had an almost 30 percent lower risk of pneumonia and hospitalization from infections. Another study published earlier this year found that gum disease is associated with a roughly 10 percent higher mortality over 10 years among patients with kidney problems.
To date, more than 500 scientific papers have weighed in on the connection between atherosclerosis and gum disease. Officially, the theory remains “biologically plausible,” but unproven, according to the American Heart Association’s formal position. Some concepts are undisputed: For one, the microbes that live in the mouth don’t stay in the mouth. The simple act of brushing allows bacteria clinging to the teeth and gums to leak into the bloodstream.
As the posters at the dentist’s office attest, neglected oral hygiene encourages bacterial growth, allowing the microbes to breed on and between teeth, as well as under the gums. What the illustrations don’t show is that these microorganisms form a biofilm, a tough microbial community bound together with sugar molecules in a thin coating. This is the plaque your dentist warns you about.
“If you do not brush your teeth, it will sit there and accumulate. As that plaque gets thicker and thicker, there is less and less oxygen in the deepest layers,” Borgnakke says. Safely sheltered, the innermost plaque starts to favor anaerobic bacteria, which, when they escape into the blood, can survive in the oxygen-starved nooks and crannies deep inside the body.
As plaque builds up, gums get irritated, swell and draw more blood into the distressed tissue. Eventually, chemicals produced by the biofilm break down the thin layer of cells that form a boundary between the gums and the blood vessels. Periodontitis officially occurs when gum and bone tissue starts to deteriorate. The space between the tooth and gums forms a pocket; dentists measure the depth of the pockets to determine the severity of infection. “We usually think of an infection as some bug from the outside that attacks the body,” says Borgnakke. “In this case, it’s an internal infection.”
It was once thought that only a handful of microbial species were involved in the development of periodontitis, but the latest studies have revealed that many of the microbes responsible for gum disease come from “previously underappreciated species,” according to a 2015 report in Advances in Experimental Medicine and Biology. Because many bacteria resist growth in a laboratory, only a small portion of some 500 to 700 species of oral microbes have been well characterized.
One aggressive pathogen, an organism called Porphyromonas gingivalis, has antennae that stick out and can pry open the space between two cells, Borgnakke says. “This is a really, really nasty bug.” Within minutes of invading blood vessels, P. gingivalis and its gang of accomplices are ferried to distant sites, where they can set up outposts. “Bacteria that normally live in the mouth are found in every organ in the body, and even muscle cells,” she says.
The body doesn’t take this assault lying down. The immune system gets agitated and tends to stay in a state of slow simmer. But the bacteria that cause periodontal disease have a knack for turning the body’s defense on its head, according to a 2015 review in Nature Reviews Immunology. Case in point: Common white blood cells called neutrophils are deployed to the failing gums — where they not only fail to control the infection, but also end up releasing enzymes that further destroy tissue. The immune system also releases an avalanche of chemicals designed to help control the infection. For example, the liver starts producing C-reactive protein, a molecule that has such an important role in signaling the rise of heart disease that it is considered a risk factor by some researchers.
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