Voyage into the great unflossed

A dental-phobic writer takes a trip into the cavity we call the mouth.

By Susan McCarthy
Published August 18, 1999 4:00PM (EDT)

For the little creatures living in your mouth, life is a dim-sum restaurant. They sit, warm and cozy, and waiters come by with carts of food to select from. True, the place is periodically raided by goon squads armed with toothbrushes, who raze whole neighborhoods, but no sooner do the squads move on than the forces of free enterprise rush back with cinderblocks and put up a whole new entertainment district.

Many creatures dwell in your mouth, composing a dense and varied ecology, diverse species flourishing in the paradise that is you. "It gets up to at least 500 species in a normal person's mouth," says Paul Kolenbrander, a research microbiologist at the National Institute for Dental Health. There are bacteria, fungi, yeasts, viruses, even a few rare protozoa shambling about in there. You've probably always longed to explore the hidden wonders of this dark, enameled Eden, to dart past gnashing incisors and come face to face with the Great Unflossed, to look the gift horse in the mouth and drop pointed hints about dentifrices. We shall make this perilous trip, with an expert guide to steer us past the worst perils. This is no tourist trap; this is the Inner Journey.

You can think of them as invisible foes -- ghoulies and ghosties -- or you can think of them by their classic name, the oral flora, the flowers of the mouth. I think of them as the denizens of a little-known coral reef, previously undiscovered by rampaging ecotourism.

Their study is neglected compared to coral reefs, which are scrutinized by scientists, entrepreneurial aquarists and legions of amateurs laden with snorkel gear, scuba gear and underwater cameras. The modern coral reef bristles with TV crews, whereas I have been unable to interest either the Discovery Channel or Animal Planet in exclusive rights to film my gums.

Yet plaque and calculus, the soft and the hard films that keep trying to form on your teeth, and that are only held at bay by your meticulous regime of oral hygiene, are indeed reef-like, and worthy of our visit.

Plaque is a structure formed by microorganisms grappling to the surface of the tooth or to the microorganisms who got there first. They are it; they build it and they live in it. All is trolleys of dumplings until the toothbrush hits. Calculus is mineralized, and also protects its fragile tenants.

The coral reef metaphor boggles my dentist.

My mind is always racing at the dentist's office, because I had dreadful childhood dental experiences, revolving around the ancient "That hurts!"/"It does not!" patient-doctor duality. I have a great dentist now, a painless practitioner. But with a lifetime's anxiety built up, I do get nervous, and my mind does whirl, and I keep asking bizarre questions. "If the bacteria in my mouth live in the plaque, which they construct themselves, aren't they just like corals building a coral reef? Are there moray eels? Are any of them striped?"

These questions seem to perturb my dentist. After all, we think of coral reefs as good and plaque as bad. Perhaps he wonders whether I'm going to campaign to Save the Old Growth Plaque. I think he thinks I'm strange.

OK, I'm strange. But suppose it is like a reef and we are tiny ecotourists snorkeling about -- or diving, in the case of creatures who live in subgingival crevices under the gums -- what would we see? We'd have to dive a lot, because 80 to 90 percent of the inhabitants of the Grand Canyons of the Fangs are anaerobic. They neither need nor can tolerate oxygen, so they hide in not-so-airy places, or under heaps of other, tougher species. Saliva, while providing a handy medium for creatures of the mouth to move around in, is not exactly hospitable, being full of immunoglobulins and other factors that make their lives difficult. Like the open sea, it is a wasteland compared to the reefs.

Our exploration will take us skimming just above the surfaces of these formidable caverns, like underwater spelunkers with a rash taste for hallucinogens.

As many as 64 species of bacteria have been cultivated from the surface of a single tooth. For wildlife viewing, it's best if the teeth haven't been brushed recently. For the reef is agitated by forces stronger than oceanic tides, fiercer than storms: the effective dentifrices of the world. These dash away the structure, obliterate populous communities and leave blasts of cruel saliva sweeping across enamel plains.

This reef is a bleached place, inhabited by pale ghostly creatures. No one there has eyes to see, so there's no advantage in dramatic coloration. Being beautiful won't attract a mate, gaudy stripes can't warn predators how toxic you taste, clever cryptic patterns won't make you seem like part of the scenery. If the denizens of the mouth had had warning that we were coming to look at them, would it have been to their benefit to be peacock-tailed bacteria, parrot-faced bacteria, purple-and-gold bacteria with eyespots? Would we be more merciful if they were more lovely? Would we stop brushing?

I don't think so either. Though we might have visited sooner.

Some residents of the mouth do happen to be colored, even if they're not tiger-striped. They come in a variety of shapes, from blobs and chains of blobs to elaborate creatures that zip around by means of whip-like flagella or that lumber about on pseudopods. Feathery colonies twine like vines over columnar, stacked colonies. Others look like plumes of whipped cream flung across a plain covered with clusters of grapes.

As our little ecotourism group flippers into the canyon lands, we glance up at the roof of the mouth and our guide (we'll assume it's an annoyingly buff microbiologist trapped in an endless doctorate) points out colonies of Simonsiella gliding about. They are flat, segmented, filament-shaped bacteria not unlike caterpillars, found on the tongue and hard palate in about 20 percent of the population. As far as we know, they are harmless fellows, not associated with infection and fond of oxygen: creatures of the open air.

But Buffy, our guide, wants us to be careful. Do not stray from the group! There, on the tongue, she points out streptococci, round blobs arranged in innocent-looking chains. There are many species of strep, some as innocent-looking as this Streptococcus salivarius, others causing strep throat and vicious tooth decay.

Buffy quotes the poetic works of J.T. Tunnock, DDS, who so memorably noted:

The Streptococcus is a most minute little creature,

Measuring about the twenty-thousandth part of a millimeter.

... Any part of the enamel not admitting hygienic action

Is invariably the seat of this monster's attraction.

It locates itself there on the surface of the tooth --

In the child, the adult, and also the youth.

One of the wickedest is Streptococcus mutans, with the power to colonize utterly smooth tooth surfaces. And sucrose is the co-conspirator -- "Nobody brought any sucrose with them, did they?" Buffy demands. Thank God no one did because it makes sticky plaque on formerly pristine enamel, then eats away at the hapless tooth. This also gives a foothold to bacteria without the ability to make glue out of sugar.

Streptococcus mutans cannot live in a mouth without teeth. When you were an infant you had no S. mutans; and if, as an adult, you had all your teeth pulled, S. mutans would vanish.

Now that Buffy feels we are properly awed, she directs our attention to yellow and white grapelike bunches of staphylococci and quotes the immortal verse of Tunnock:

Of Staphylococci pyogenes, four main classes are seen,

The orange, the white, the yellow, and green.

(Buffy apparently believes that's poetry. There really are two cultures, aren't there?)

- - - - - - - - - - - -

Before us now are branching Actinomyces bacteria. Seeing them is a special treat, for they live nowhere else but in mouths. Over there, a Wolinella is speeding about by lashing its flagellum. The pool cues are Bacterionema, a possibly villainous generator of plaque. Also dangerous are the gliding gray, pink and yellow rods of Capnocytophaga. Those flattened beans are Neisseria, bacteria that do not reproduce well in captivity, so that microbiologists like Buffy call them "delicate" and "fastidious."

The filaments of Veillonella alcalescens are more friendly -- they don't get along well with Strep mutans, or at least they may reduce their ability to make cavities, and isn't the enemy of my enemy my friend?

Buffy steers us away from the black rods of Bacteroides, associated with periodontal disease. Their weakness is Vitamin K -- they can't make their own, so they have to get it from some other species. Buffy takes a fistful of vitamins from her belt pack and allows us to photograph her tossing them showily to the Bacteroides. (She seems to fancy herself as the Jane Goodall of the gums, down to the khaki shorts and ponytail.)

The nimble helices are spirochetes, extraordinarily mobile, flexing and creeping over solid surfaces. They too are rare in captivity, but they're worth keeping an eye on, since they're implicated in periodontal disease.

Buffy becomes excited, spotting Trichmonas, a huge ponderous thing with a topknot of flagella. Like an elephant with a wig -- wait, there are no elephants on coral reefs -- like a giant sea tortoise with a wig, Trichmonas is benign and photogenic. While Buffy is passing out handouts about fungi, I will slip over and give it a kiss.

There! I take a superior attitude to Buffy's warnings about fungi, since I want to be sure everyone on the tour knows that I know all this already.

Bacteria seem to keep fungi at bay, notably various species of Candida. The best known, Candida albicans, a cream colored fungus that likes your tongue, is found in small number in 45 percent of adults. In babies and in immunocompromised people it sometimes flourishes under the old-fashioned name thrush. Adults with sturdy immune systems sometimes get thrush after prolonged use of antibiotics, which kill off the bacteria and let the fungus run amok.

My dearest wish is to see a tooth amoeba, Entameba gingivalis, a big, lumbering thing that gets about by extruding pseudopodia (temporary tentacles or, literally, fake feet; it is the giant squid of the dental deeps). Buffy has spoken optimistically of recent sightings. These harmless commensal creatures are found, in small numbers "in over 30 percent of ostensibly hygienic mouths," one textbook notes. (I'm relieved that Tunnock knew them not.) They are apparently scavengers, so while they're often seen in cases of periodontal disease, they don't cause it, any more than a vulture kills its own dinner. But Buffy's all talk: No tooth amoeba shows itself.

I was thrilled when I first heard of tooth amoebae, and asked my dentist about them, hoping he had a terrarium full of the things. But not only had he never seen amoebae bounding over my or anyone else's teeth, he was skeptical of their existence.

When I called Dr. Kolenbrander at the NIDH, he restored my faith. "I have seen them," he told me.

As far as I am concerned, tooth amoebas should be worshipped like pandas, as the charismatic megafauna of the mouth. (At tour's end, Buffy offers to sell us T-shirts that she says have the image of an amoeba on it -- life-size. As if I would fall for that old trick.) But as my dentist's skepticism shows, not only do relatively few nature enthusiasts take the dental ecotour, most dentists do not spend much time peering at and identifying the microorganisms they deal with. "Kill them all and let God sort them out," is their view.

- - - - - - - - - - - -

It wasn't until the 1950s that it was proved that bacteria cause cavities. Proof came from germ-free rats delivered by Caesarian section, hand-fed sterile food and housed in isolation chambers, breathing filtered air. There was not a cavity among them. Unless you fed them bacteria. Interestingly, their pristine lives didn't protect the rats from periodontal disease.

Scientists are just beginning to look at the interactions of the oral flora. Sig Socransky and colleagues, ecologists of the mouth, have described "microbial complexes" in plaque below the gums. Certain species are found together: If you find Porphyromonas gingivalis, you may be sure Bacterioides forsythus is close by.

They examined 185 "periodontally healthy" people all with "at least 20 teeth" and did cluster analyses of the species found. They found several complexes, which they designated by colors. The red complex, which includes the dynamic duo of Porphyromonas gingivalis and Bacterioides forsythus as well as the sinister Treponema denticola, is a gang you would prefer not to have dominating your mouth, strongly associated as they are with gums that bleed when probed by experimenters and other indicators of suffering.

The authors speculate that the bacteria of the orange complex may pave the way for the depredations of the red complex, but this is still uncertain. Others, such as the green and purple complexes, seem relatively innocent, uncorrelated with disease.

There's speculation that there are healthy microflora and unhealthy microflora. (That's healthy for us, not for them.) There may be antagonistic relationships between different complexes of bacteria, so that they are busy repelling each other, poisoning each other, and generally oppressing one another. If we can figure out the internal politics, we might support the ones that do less damage to our teeth, and wreck the prosperity of the more destructive species, dental researchers hope.

It's true that our record on manipulating ecologies (like polities) includes some spectacular bungling. On the other hand, the simple act of brushing teeth might be likened to having beneficial forest fires. (I am aware that coral reefs don't have forest fires and that dim-sum restaurants seldom do. This is a new metaphor, OK?)

A freshly cleaned tooth gradually acquires plaque with a few microbial species in it. "Pioneer communities," ecologists call these. With time, more and more species put on sunbonnets and join in. They alter their environment, lowering the pH and using up the oxygen so anaerobic species flourish. It takes just three days to go from a monolayer of bacteria to towering columnar palisades of bacteria. In these climax communities, there are hundreds of species, including exotic rarities. Decadent aesthetes are seen. Ecotourism booms! Buffy takes out an ad!

Then you suddenly remember about dental hygiene, you brush and floss, maybe even go to the dentist, and there's a new frontier. Aesthetes are replaced by brave pioneers in calico and rabbitskin.

Now that you're acquainted with the full dimensions of the horror, you may be filled with killing zeal. You may wish not merely to rush into the dim-sum restaurant, kicking over tables, overturning trolleys, but to put the place out of business once and for all, perhaps by poisoning all the customers.

This is probably not feasible, unless you want to try for the germ-free life. A newborn baby's mouth is colonized by oral flora within hours of birth. As for the germ-free lifestyle, the evidence of rats would seem to indicate it's not all that appealing, involving as it does shrunken hearts, lowered blood volume and "chronic mild diarrhea." And no frequent flyer miles.

You might try to kill all the bacteria with powerful antibiotics, but that wouldn't last long. "I don't think it's possible, and I don't think it's useful," Dr. Kolenbrander says. The minute you kill off the bacteria, the fungi make a lunge for the Oval Office and the red phone, and then you've really got problems.

It's our human habit in examining ecologies to pick out heroes and villains, good species and bad species. But periodically we're forced to rehabilitate villains who turn out to be heroes, and to pass lightly over former hero species who ate our lunch when we weren't looking. We learn to applaud scavenger species like vultures for keeping meadows clean, spiffy and odor-free.

So it behooves us to move cautiously before we tamper with the oral ecology on a species-by-species basis. Not only is it likely that bacterial complexes keep each other in check, not only is seeking the germ-free life impossibly unfun, but it may be that the oral flora keep our immune systems strong by regularly stimulating the immune response. Germ-free animals have terrible immune systems. You can knock them out with the feeblest pathogen.

For the time being, respect for ecosystems coupled with the controlled-burn approach of normal dental hygiene (I brushed my teeth so frequently while researching this story that the things are worn to wafers) seems like the most viable way to treat the oral flora. And if ecotourism expands to include the coral reefs of the mouth, Buffy will finally be able to flee academia and get a real job with benefits, maybe even a dental plan.

Susan McCarthy

Susan McCarthy is a San Francisco freelance writer and the author, with Jeffrey Masson, of "When Elephants Weep: The Emotional Lives of Animals."

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