PARIS (AFP) - Here's another reason to brush your teeth: poor dental hygiene boosts the risk of heart attacks and strokes, a pair of studies reported this week.

Heart disease is the number one killer worldwide, claiming upward of 17 million lives every year, according to the World Health Organization.

Smoking, obesity and high cholesterol are the most common culprits, but the new research shows that neglected gums can be added to the list.

"We now recognize that bacterial infections are an independent risk factor for heart diseases," said Howard Jenkins of the University of Bristol in Britain, at a meeting of the Society for General Microbiology in Dublin.

"In other words, it doesn't matter how fit, slim or healthy you are, you're adding to your chances of getting heart disease by having bad teeth," the professor said.

There are up to 700 different bacteria in the human mouth, and failing to scrub one's pearly whites helps those germs to flourish.

Most are benign, and some are essential to good health. But a few can trigger a biological cascade leading to diseases of the arteries linked to heart attacks and stroke, according to the new research.

"The mouth is probably the dirtiest place in the human body," Steve Kerrigan of the Royal College of Surgeons in Dublin said.

"If you have an open blood vessel from bleeding gums, bacteria will gain entry to your bloodstream."

Once inside the blood, certain bacteria stick onto cells called platelets, causing them to clot inside the vessel and thus decreasing blood flow to the heart.

"We mimicked the pressure inside the blood vessels and in the heart, and demonstrated that bacteria use different mechanisms to cause platelets to clump together, allowing them to completely encase the bacteria," he said.

This not only created conditions that can provoke heart attacks and strokes, it also shielded the bacteria from both, immune system cells and antibiotics.

"These findings suggest why antibiotics do not always work in the treatment of infectious heart disease," Jenkins said.

In separate research, a team led by Greg Seymour of the University of Otago Dunedin in New Zealand showed how other bacteria from the mouth can provoke atherosclerosis, a disease that causes hardening of the arteries.

All organisms -- including humans and bacteria -- produce "stress proteins," molecules produced by conditions such as inflammation, toxins, starvation, or oxygen deprivation.

One function of stress proteins is to guide other proteins across cell membranes.

But they can also can latch onto foreign objects, called antigens, and deliver then to immune cells, provoking an immune reactions in the body.

Normally, the body does not attack its own stress proteins.

But bacterial stress proteins -- which are similar -- do trigger a response, and once that has happened the immune system can no longer differentiate between the two, said Seymour.

"White blood cells can build up in the tissue of arteries, causing atherosclerosis," he explained in a phone interview.