Understanding of the relationship between oral health and general health bolsters the tried-and-true advice to brush, floss, and see your dentist often.

Only a generation or so ago, most people who lived long enough lost their teeth to decay and periodontal (gum) disease. Today, toothlessness (edentulism) is on the decline. Most of us can expect to keep all or most of our natural teeth for the rest of our lives, and our children and grandchildren have a good chance of avoiding cavities as well as tooth loss.

In part, the improvement in Americans' oral health is due to the addition of fluoride to drinking water, a practice that began after World War II and dramatically reduced tooth decay among children. Another breakthrough came a couple of decades later, when researchers discovered that bacterial infection causes both tooth decay and gum disease. Suddenly, it became possible to counter the two major causes of tooth loss with one strategy: reducing bacterial levels in the mouth. Fortunately, most of us can accomplish this with brushing, flossing, and regular dental care.

Although tooth loss isn't the threat it used to be, we're by no means home free. Nearly 80% of people over age 65 have periodontal disease, much of it asymptomatic.

The significance of oral disease extends beyond the mouth. Research conducted in recent decades has uncovered potential links between chronic gum infection and conditions as serious as diabetes, stroke, heart disease, and pregnancy complications. Also, oral diseases may contribute to malnutrition and create a variety of psychosocial difficulties, including low self-esteem and discrimination in work or social settings. Scientists are also looking at whether hormonal changes at menopause increase the risk for gum disease and tooth loss in women.

How trouble starts

Decay-producing bacteria mix with saliva to form plaque, a sticky film that adheres to the surface of the teeth. The bacteria consume sugar from bits of food left in the mouth and excrete lactic acid, which becomes part of the plaque layer. If plaque isn't removed, the acid dissolves the tooth's enamel (hard outer coating) and inflames the gum tissue.

Plaque that forms on easily accessible surfaces can be dislodged with natural chewing and tongue movements. But in hard-to-reach places - between the teeth, in the furrows of the molars (back teeth), and at the edges of the gums - plaque can build and start disease.

Most people can keep bacteria in check with a relatively simple regimen of home care and regular dental checkups (two per year are usually enough). However, some individuals have less natural resistance to oral bacteria and make little headway against decay and gum disease despite their best efforts. A dentist can help identify people with this problem and individually tailor their oral care.

Tooth decay

Harmful bacteria in the mouth, especially Streptococcus mutans, or S. mutans, thrive on a steady supply of carbohydrates, especially sugars. Poor oral hygiene enables them to feed and grow without interference. The acid the bacteria produce when they metabolize sugar can demineralize (dissolve) the tooth surface. Ordinarily, this process takes place slowly, giving the body time to replenish the enamel.

However, bacterial accumulation can outpace the body's ability to rebuild enamel, resulting in the first stage of decay (dental caries), which appears on the tooth as a spot discernible only to a dentist or on an x-ray. The dentist may be able to arrest the process at this stage by applying either fluoride to boost remineralization, an antiseptic to reduce the level of S. mutans in the mouth, or a sealant on the molars to create a physical barrier against bacteria.

Once decay has bored a hole in the enamel, a cavity forms, and the tooth can no longer repair itself. A dentist will need to clean out the cavity and fill the tooth.

Researchers funded by the National Institute of Dental and Craniofacial Research announced in October 2002 that they now have the genetic blueprint of S. mutans. This discovery may lead to treatments that interfere with the bacterium's activity. For example, targeting its adherence genes could keep it from sticking to teeth. Scientists are also trying to develop a modified and harmless strain of S. mutans to function as a vaccine, spurring the body to produce antibodies against the disease-causing strain.

Periodontal disease

Gum disease gets a foothold when plaque forms in the sulcus, a shallow trough at the point where the gum meets the tooth. Without proper cleaning, plaque and additional bacteria can build up here, creating a tiny pocket where the gum tissue pulls away from the tooth. Toxins released by the bacteria inflame the surrounding tissue. Meanwhile, the surface of the plaque hardens into calculus, or tartar, which further inflames the gums. This condition is called gingivitis.

Gingivitis can be corrected if caught early. The first step is a thorough professional cleaning to remove plaque and calculus, along with better brushing and flossing habits. Most gingivitis is the result of poor oral hygiene, although some rare forms - for example, desquamative gingivitis, which affects primarily postmenopausal women - are not caused by plaque accumulation.

Left untreated, gingivitis can progress to periodontitis. Here the pocket widens as the gum pulls back from the root of the tooth. The disease also destroys the periodontal ligament and bone, eventually causing the tooth to loosen or fall out.

Treating periodontitis usually involves removing the accumulated plaque and calculus above and below the gum line with a procedure called scaling. It's generally performed under local anesthesia. A periodontist may also need to surgically remove damaged gum tissue. Antibiotics may be used to reduce bacteria and inflammation. After initial treatment, the patient must brush and floss without fail and visit the dentist every three months.

The fundamentals

People often wonder if there are preferred methods for brushing and flossing. There are several techniques, and the choice depends on particular dental conditions and oral anatomy. All of them have the same goals: removing food, stimulating gums, and preventing plaque buildup. (For brushing and flossing styles that work for most people, please visit our web site, http://www.health.harvard.edu/newsletters/Harvard_Womens_Health_Watch.htm

A wedge stimulator (a small pointed stick with one flat side) can also help remove plaque and food particles between teeth and reduce gum inflammation and bleeding.

• When to brush. To prevent plaque buildup, it's best to brush at least twice daily - after breakfast in the morning and before bedtime at night. Replace your toothbrush when the bristles splay out of line, every three months or so.

• Type of toothbrush. Most dentists recommend soft-bristle brushes. A hard-bristle brush can damage the gums and teeth if applied too vigorously or used with abrasive toothpaste. Rounded bristles are less likely than blunt-cut bristles to damage gum tissue. And the best overall shape of the bristle surface is flat or multilevel, as opposed to rounded or curved.

• Should I use an electric toothbrush? Used properly and consistently, most manual and electric toothbrushes achieve similar results. But one particular type of electric toothbrush may offer some advantages. Researchers with the Cochrane Collaboration, a nonprofit group based in England, reviewed 29 published studies conducted between 1965 and 2001 that compared the results of manual and power brushing. They found that an electric toothbrush whose bristles spin first in one direction, then another - called "rotation oscillation" action - removed 7% more plaque and reduced gum disease 17% more effectively than a manual brush. (The Cochrane Library, Issue 1, 2003).

An electric toothbrush may also be a good choice for people who have trouble reaching their back teeth.

• What kind of toothpaste? Toothpaste is typically a mixture of abrasives, foaming agents, water, and binders, with flavor, color, and sweeteners added. Most contain fluoride. An antibacterial agent, triclosan, has long been used in Europe to combat gingivitis. Colgate Total is the only FDA-approved brand in the United States that contains this agent.

An important consideration is level of abrasiveness. Unless your teeth have tough stains, choose a low-abrasive toothpaste. (For a chart showing the relative abrasiveness of popular brands of toothpaste, visit our web site, http://www.health.harvard.edu/newsletters/Harvard_Womens_Health_Watch.htm

• Should I use a rinse? Mouth rinses are either cosmetic, like a mouthwash, or therapeutic - for example, rinses that fight plaque or control bacteria. A mouthwash can kill harmful bacteria, but doesn't eliminate them altogether, and bacterial levels rebound quickly. Although a mouthwash may leave the mouth feeling fresher, it has no lasting effect on tooth decay or gum disease.

Some therapeutic rinses contain fluoride for cavity prevention. The only over-the-counter rinse that effectively fights plaque is Listerine. Long-term studies have shown that rinsing twice a day with Listerine or a generic equivalent can reduce plaque buildup and gum inflammation by 34%.