Oral Exams
The next time you kiss someone, think about this: In your mouth, and in the mouth of every adult, live more than 400 species of microorganisms, mostly bacteria.
Billions and billions of them growing in layers, crowded together and wrapped cozily around each other, on every slimy surface, dark nook and inviting cranny.
It’s enough to make you keep those lips permanently pursed.
“In one mouth, the number of bacteria can easily exceed the numbers of people who live on Earth,” says Dr. Sigmund Socransky, a dental researcher at the Forsyth Dental Center in Boston. “In a clean mouth, 1,000 to 100,000 bacteria live on each tooth surface. A person who has not a terribly clean mouth can have 100 million to 1 billion bacteria growing on each tooth.”
With an average temperature of about 95 degrees Fahrenheit, humidity of 100%, abundant oxygen and regular stoking with sugar and simple carbohydrates--manna from bacterial heaven--the mouth provides a home for a diversity of species.
But these facts are more than just useful fodder for cocktail party chatter. An entire branch of dental research is growing up around oral ecology to develop the next generation of weapons in the fight against tooth and gum disease.
Since 1959, when scientists isolated a species of bacteria that causes most cavities, a national campaign to reduce tooth decay focused on a combination of brushing, flossing and adding fluoride to water supplies, toothpaste and mouthwashes. These dental hygiene methods have worked so well that 50% of American children younger than 12 have no tooth decay.
But of the remaining 50%, many have severe forms of decay that are difficult to control, even with the best dental hygiene. And periodontal disease, which is caused by about half a dozen bacterial species, affects millions of people, mostly adults.
Over the last 20 years, modern biological technology, including genetic engineering and techniques to study anaerobic bacteria--those that live without oxygen and cause most periodontal disease--has enabled scientists to identify many of the organisms. But it’s an ongoing battle.
Oral ecologists such as Socransky have determined that the mouth’s microorganisms have evolved with the human species.
In exchange for living in their tropical paradise, the mostly beneficial bacteria help fend off disease-producing bacteria that attempt to infiltrate the mouth from the outside world. But researchers funded by the U.S. National Institute of Dental Research have pinpointed about a dozen species of bacteria and yeast that can cause infections in the teeth and gums of virtually every human in the world.
In the last five years, they have made significant strides in understanding how these organisms are transmitted and are in the initial stages of developing startling new methods of dental care to prevent the spread of infection.
*
When human babies pop into this world, their wails of greeting burst from sterile mouths.
“Within minutes to hours, they are colonized with organisms that stay with them until they die,” Socransky says.
The bacteria, yeast, viruses and protozoa--most of which are beneficial--enter a baby’s mouth from saliva drops spewed into the air, a breast or bottle nipple.
The growth of organisms in the mouth follows the classical pattern of ecological succession, the same way bare land eventually turns into thick jungle. A few pioneer species settle first, creating a habitat friendly to other species, which move in.
When the first baby teeth push through the gums, another set of species--including the dreaded Streptococcus mutans, the bacterium believed responsible for most tooth decay--crowds in. During puberty, the composition of saliva changes, so that another group of organisms immigrate and flourish.
By adulthood, mouths harbor what’s known as a climax community--a complex group of organisms, each with its own preferred microhabitat.
Some species live only on the cheeks. Others prefer the back of the tongue over the front, especially the group of anaerobic bacteria that live in the crevices of the tongue and emit hydrogen sulfide, the origin of most severe bad breath. Another group will survive only on the palate.
The teeth themselves provide a plethora of living options--surfaces that face the outside world, sides with a view of the back of the mouth, a strip along the edge of the gums, or the gloomy, wet oxygen-deprived spaces between the gums and the teeth.
“Seeing all these different forms of life and figuring out what they do and where they live is fascinating,” Socransky says. “You get a sense of what kind of world is living with you.”
*
Ecological conditions in the mouth are never stable. People change their diets, they lose teeth, they have crowns or false teeth put in. Changes occur after every meal, every brushing and flossing, even after every swallow, as millions of bacteria die or lose their hold on teeth and tumble down the throat.
During a night’s sleep, when saliva production drops to near zero, bacteria revel in their freedom and multiply with abandon until the dawn. (Saliva, the magic fluid that keeps this ecosystem in balance, harbors its own collection of bacteria.)
Most of the time, however, the inhabitants of the mouth live in more or less perfect harmony.
“Congress should take lessons from the mouth,” says Dr. Yolanda Bonta, manager of clinical research in the department of research and development at Colgate Oral Pharmaceuticals in Piscataway, N.J.
But, like this winter’s breakdowns in budget talks that led to the shutdown of the government, sometimes things go to hell in a handbasket in the mouth too.
For example, abundant sugar sends S. mutans into a frenzy of activity. The bacteria spew acid and overtake saliva’s ability to buffer the acid, which eats away at the minerals of tooth enamel. Without adequate brushing and flossing, plaque grows, producing calcified deposits and a cozy home for more species that do more damage. A microscope reveals the plaque you scrape off your tooth with your fingernail as a daunting tangle of thick vines ending in corncobs winding in and around a slowly writhing mass of lumps and worm-like creatures.
Stress, medications or other unknown factors can also throw the ecosystem of the mouth out of whack, causing a species of bacteria that keeps periodontal disease-producing bacteria under control to disappear and the gums to turn red with infection.
More than 200 types of medications, as well as radiation of the head and neck for cancer treatment, cause a drastic drop in saliva production, and bacteria run rampant.
Bacteria, yeast and protozoa that dive down tiny cracks in a tooth to infect the blood- and nerve-rich root cause ice-pick-like stabbing pain. During a dental procedure, they can get into the bloodstream or the brain, where they can cause madness and death.
Although procedures for root canals were known for hundreds of years, it was not until after World War II, when antibiotics were developed, that dentists were allowed to perform root canals in the United States, because bacterial infection in the root was so difficult to contain.
“That’s why eye teeth are called eye teeth,” says Dr. Kathleen Olender, an endodontist at UC San Francisco Dental School. “If that particular tooth gets infected, the bacteria can easily travel to infect the eye.”
*
In the last few years, researchers have made significant strides in understanding the bacteria and how they are transmitted from person to person. One important discovery was that S. mutans consists of thousands of strains, some that are more harmful than others, which partly explains why some people have more or more severe cavities than others.
At the University of Alabama School of Dentistry, Dr. Page Caufield, a professor of oral biology, found that humans are colonized by S. mutans during a “window of infectivity,” around 2 years of age.
At that time, S. mutans is passed from the primary caregiver--usually the mother--probably through saliva transmitted when minuscule droplets are spewed by talking into the face of a child whose teeth are emerging.
“The window opens and closes,” Caufield says. “If children aren’t infected by S. mutans then, another bacteria species colonizes and uses that niche. People don’t exchange S. mutans as adults.”
But recent studies indicate that bacteria that cause periodontal disease are passed between spouses and “significant others,” says Dr. Ernest Newbrun, a dental researcher at UC San Francisco Medical Center.
“It isn’t clear that that’s the only way those bacteria are transmitted, but it seems to require direct mouth-to-mouth contact--in other words, kissing.”
This knowledge is leading scientists to develop new methods of preventing diseases of the teeth and gums.
“Instead of trying to kill all the microorganisms, we’re targeting specific interactions,” says Dr. Irwin Mandel, recently retired from decades of research at Columbia University School of Dental and Oral Surgery. “It’s more of a magic-bullet approach than a shotgun approach.”
In Alabama, Caufield and his team will soon begin clinical trials on 250 women who carry very harmful strains of S. mutans. Their teeth will be treated with an antiseptic and varnished during their children’s window of infectivity. The scientists hope that the trialswill result in 250 children who will live a life free of the S. mutans strains that have plagued their mothers.
At the University of Florida at Gainesville, molecular biologist and dentist Dr. Jeff Hillman searched through hundreds of strains of S. mutans and found one that produced an antibiotic-like molecule that kills all other strains of S. mutans.
In addition, other research includes the development of artificial saliva for the 2 million people with Sjogren’s syndrome, an autoimmune disease that causes severe drying of the mouth, eyes and other mucosal surfaces.
Another group of scientists is exploring how to block bacteria from using their unique adhesives to attach themselves to teeth. And still others are trying to develop vaccines against the bacteria that produce cavities and periodontal disease.
These new methods are still five to 10 years away from seeing the inside of a dentist’s office, researchers say. Until then, Bonta, of Colgate Oral Pharmaceuticals, advises going no more than 72 hours without brushing. “If you pass that threshold, you may not be able to remove the plaque and bring the infected parts of the teeth back to health.”
And it’s best to brush for more than a minute, says UCSF’s Newbrun. Fifty or 60 seconds isn’t enough to clean the 150 tooth surfaces that most of us have, to bring that bacteria count down to a manageable and healthy 1,000 to 100,000 per tooth.
