Bacteria are believed to cause Alzheimer's, Parkinson's and other diseases
For decades, health experts have been telling us about our bad habits, blaming them for the increase in "lifestyle-related diseases". These often occur with age and include heart disease, Alzheimer's disease, type 2 diabetes and some cancers. Worldwide, 70% of all deaths are now attributed to these conditions.
Are bacteria the cause of many diseases?
Too much red meat, too little fruit and vegetables, smoking, alcohol consumption, obesity and lack of exercise seem to make all these diseases more likely. But no one really knows why, and we still haven't found the cause of these problems.
Alzheimer's is now one of the leading causes of death and the number of people affected is expected to rise from 35.6 million to 65.7 million in 2030, but no researcher knows exactly where it comes from. Another example: high blood cholesterol levels are believed to be the cause of heart attacks, except that most people who suffer from them do not have high cholesterol levels.
What we do know is that these conditions generally begin to cause symptoms later in life, and their prevalence increases dramatically as we live longer. They all turn to inflammation, the method used by our immune system to kill invaders against us. And, by definition, these diseases are not communicable. It would be bad from our genes.
But there is a new lead: one disease after another, we discover that bacteria are secretly involved, invade organs, strengthen the ability of our immune system to improve their own survival and gradually destroy us. The implication is that we could eventually defeat heart attacks or Alzheimer's disease simply by killing these germs.
The involvement of bacteria was completely out of our control
Until now, the involvement of bacteria has escaped us completely. This is because they tend to work very slowly, to remain dormant for long periods of time or to hide in our cells. This makes them difficult to grow in the laboratory, to bind bacteria to diseases. But now, DNA sequencing has revealed the presence of bacteria in places where they were never supposed to be, manipulating the inflammation observed in all these diseases.
The results are so contrary to the scientific literature and are emerging in many diseases, each with its own research community, that awareness of all this is only beginning to reach the general public. And as expected, as with any paradigm shift, there is resistance.
But some researchers, frustrated by years of failure to find the causes, and therefore real potential treatments, of diseases of aging, are cautiously excited. And with good reason: it could change everything.
The bacteria responsible for gum disease are the worst responsible, and they seem to play a role in the greatest number of diseases. It is the most common disease of aging. In fact, it is "the most common disease in humans," says Maurizio Tonetti of the University of Hong Kong. In the United States, 42% of people aged 30 or older have gum disease, but this percentage rises to 60% among those aged 65 and over. It was measured at 88% in Germany.
The main hypothesis of the birth of Alzheimer's disease
It is striking to note that many age-related diseases - from rheumatoid arthritis to Parkinson's disease - are more susceptible, more serious, or both, in people with gum disease. It is possible that a third thing could go wrong, leading to both gum disease and other diseases. But it is increasingly clear that the relationship is direct: the bacteria behind gum disease help to cause others.
The circumstantial evidence is certainly overwhelming. In the United States, which allocates federal Medicaid funds to dental care costs, including those related to the prevention or treatment of gum disease, ultimately pay between 31 and 67% less than states that do not have any, to help these people later with heart attacks, diabetes, strokes and cancer. Private insurance companies report similar trends, says David Ojcius of the University of the Pacific in San Francisco.
But how can the bacteria that cause gum disease play a role in all these conditions? To answer this question, we need to look at how they upset the immune system.
Your mouth is home to more than 1000 species of bacteria, in a stable community where bad bacteria are controlled by the good bacteria that surround them. Elsewhere in the body, including on the skin or mucous membrane of the intestine, communities of bacteria live on a continuous layer of cells, where the outermost layer is constantly eliminated, thus eliminating invasive bacteria. But your teeth can't protect themselves so easily, says Tonetti. Bacteria live on a hard surface that passes through the outer protective sheet of the cells.
When the plaque on which the bacteria live on the teeth accumulates enough to harden and spread under the gum, it triggers inflammation: immune cells flood and destroy these germs and our own infected cells. If this lasts too long, a poor oxygen pocket develops between the gum and the tooth. A handful of bacteria take advantage of this to multiply. One of them, Porphyromonas gingivalis, is particularly insidious and disrupts the stable bacterial community and prolongs inflammation.
Most pathogens try to block or avoid inflammation, which normally kills them before it goes out again. From the age of 30 or 40, this cessation begins to decrease, leading to chronic inflammation related to aging diseases and no one knows why.
P. gingivalis can participate. In fact, it perpetuates inflammation by producing molecules that block some, but not all, inflammatory processes," says Caroline Genco of Tufts University in Massachusetts. The resulting inflammation never completely destroys the bacteria, but continues to try, killing our own cells. Debris is a treat for P. gingivalis who, unlike most bacteria, needs to eat protein.
Destruction also releases the iron that bacteria need and that the body normally keeps locked in ferritin. "These bacteria manipulate their interactions with the host's immune response to improve their own survival," says George Hajishengallis of the University of Pennsylvania.
Gum control
Finally, the infected tooth falls out - long before P. gingivalis escapes into the bloodstream. Here, your immune system makes antibodies that usually protect you from germs. But anti-P. gingivalis antibodies seem to be more a mark of his passage than a protection. People with these antibodies are actually more likely to die in the next decade than those who do not and are more likely to suffer from rheumatoid arthritis, heart attack or stroke.
This could be because once in the blood, P. gingivalis modifies its surface proteins so that it can hide inside the white blood cells of the immune system, explains Genco. It also penetrates into the cells that line the arteries. It remains dormant in these places, rising from time to time to invade new cells, but remains hidden from antibiotics and immune defences. However, even when trapped in our cells, P. gingivalis continues to activate or block different immune signals, even modifying the gene expression of a blood cell to migrate it to other inflammation sites, where the bacteria can exit and feed.
One of the reasons why gum disease makes us more susceptible to diseases such as diabetes and Alzheimer's disease is that it adds to our overall "inflammatory load". But P. gingivalis can also act more directly: this bacterium has been detected in inflamed tissues of the brain, aorta, heart, liver, spleen, kidneys, joints and pancreas in mice and, in many cases, in humans.
The long term
No official medical advice to prevent these diseases includes "consulting your dentist", at least not yet. "Periodontal disease should be better recognized by the community as a clearly established risk factor," says Dominy. One of the most obvious risks is Alzheimer's. But the World Health Organization's (WHO) guidelines to prevent Alzheimer's disease published in May say nothing about preventing gum disease.
"It may be too easy to make fun of the idea that flossing can contribute to good brain health," says Margaret Gatz of the University of Southern California. And this may explain in part why this idea has not yet taken off in traditional medicine. "There is a history of dental and medical doctors working separately and not cooperating," says Thomas Kocher of Greifswald University in Germany.
But it also reflects the long-held belief that heart attacks and other conditions are mainly the result of a poor lifestyle, and that bacteria have nothing to do with these diseases. Such underlying scientific paradigms can take decades to change.
This happened when it was shown that a bacterium (Helicobacter pylori), and not stress and stomach acid, caused stomach ulcers. After decades of explanation, many medical experts are reluctant to admit that amyloid may not cause Alzheimer's disease and that high cholesterol levels may not lead to heart disease.
With the ageing of the world's population, there are only a few decades left before these diseases become a health crisis serious enough to threaten health systems and societies. We need a new paradigm. This means that we have to face the possibility that all these diseases may ultimately be caused by bacteria.
Synthesis of bacterial theory
Rheumatoid arthritis: P. gingivalis is present in the joints of people who contract this disease before the first symptoms appear and is the only bacterium known to produce a chemical substance involved in this disease.
Parkinson's disease: P. gingivalis and its protein synthesis enzymes, gingipain, are found in the blood of people with Parkinson's disease and promote the inflammation and abnormal coagulation observed.
Kidney disease: Gum disease is associated with chronic kidney disease and treating the gums seems to help the kidneys.
Foie gras disease: There is much more P. gingivalis in affected livers than in healthy livers, and the disease worsens in mice. Treating the gums seems to help.
Cancer: A bacterium has been discovered in cancers of the mouth, esophagus, stomach and pancreas in the early stages, and modifies cell functions in a way typical of these cancers.
Macular degeneration: The injection of bacteria into the retina appears to damage vision by producing age-related macular degeneration in mouse studies.
Preterm births: Gum disease, caused by P. gingivalis, has been established as a risk of premature birth.
Alzheimer's disease: Again, P. gingivalis seems to play a decisive role.
