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A winning smile can melt even the coldest of hearts, they say. But your smile might do more than just cheer people up: it may also reveal how healthy you are … or whether you're a step away from a heart attack.

Research suggests that bacterial scum in and around poorly cleaned teeth, if allowed to build over many years, not only causes infections in the mouth but could also cause serious health problems elsewhere in the body.

It could lead to a build-up of fatty deposits in blood vessels and, eventually, a heart attack or stroke. It now seems that all that brushing and flossing could actually save your life.

Heart disease is just one of a growing number of diseases, long thought to have predominantly genetic or environmental causes, which may in fact have a pathogen such as a bacterium or virus at their root. New studies of these micro-organisms are reshaping our fundamental understanding of how illnesses as diverse as heart disease, cancer and even obesity may arise.

The diverse microbial soup that resides inside our body, and its health implications, has been studied for more than 30 years by medical scientist Trevor Marshall of Murdoch University in Perth, Western Australia.

He has advanced a controversial hypothesis that bacterial infections can lead to certain chronic diseases, including many kinds of inflammations caused when the body attacks its own tissues.

The premise is based on the idea that our body is a 'superorganism', comprised of 10% human and 90% bacterial cells. Human saliva, for example, contains hundreds of different species of bacteria, as do our digestive tracts.

In 5% of cases, samples from prosthetic hip joints removed from patients revealed highly unusual bacteria more usually found in hydrothermal vent communities.

But how do these hitchhikers cause chronic disease? Marshall believes that molecules produced by bacteria directly interfere with genes responsible for producing immune system proteins.

"Take, for example, a gene called PTPM 22 that is associated with lupus, rheumatoid arthritis and diabetes [all autoimmune diseases]," he says. "If the body senses the presence of bacteria known as mycobacteria, it upscales production of PTPM 22 ,which then drives these particular diseases."

Researchers from the University of Buffalo in New York, USA, reported findings at the International Association for Dental Research General Session in April 2009 that heart attacks are more common in people with both a greater number and a greater variety of species of bacteria in their mouths.

"This does not come as a surprise, if the body cannot control bacterial levels then disease mechanisms take over," says Marshall.

As the world's number one killer, the causes and risk factors of heart disease have been widely studied. According to the World Health Organisation, each year almost 20 million people worldwide die of cardiovascular disease, a term that covers heart attacks, strokes and other blood vessel disorders.

In Australia alone, they resulted in 34% of all deaths in 2006. The upshot is that, with figures such as these, even a small improvement to our knowledge about the underlying causes could make a very big difference to the number of people affected by it.

Pauline Ford at the University of Queensland in Brisbane is studying the links between periodontal infections of the teeth and gums and heart disease. She believes that adapting our lifestyles to lessen the risk is a very effective strategy; but it is important to clarify exactly what those risks are.

"Up to 50% of people with heart disease have none of the conventional risk factors - for example high blood pressure or high cholesterol - so we need to look into other causes such as infection," says Ford.

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