Chlamydia is a bacterium that infects the reproductive and urinary tract organs of both men and women.

Researchers have produced evidence that the bacterium can trigger an auto-immune response in some patients who become infected, leading to inflammation of the heart.

Several epidemiological studies, which track the occurrence of disease in large populations, have shown a correlation between Chlamydia infections and heart disease.

But until now the mechanism that might link the two has remained a mystery.

System malfunction

Chlamydia has already been linked with female infertility, eye infections and childhood pneumonia.

Writing in the journal Science, Kurt Bachmaier, of the Ontario Cancer Institute and University of Toronto, and his colleagues in Canada, Austria, and the US, say they have evidence that Chlamydia stimulates the immune system to malfunction.

Some disease causing agents carry proteins on their surfaces that are almost identical to proteins on the host's cells.

These proteins enable the pathogens to evade the host's immune system by "passing" as part of the host's own body.

Trigger response

Sometimes, however, the immune system is not fooled by this subterfuge and launches an attack.

An auto-immune disorder arises when the attack damages the host's cells in the process.

Mr Bachmaier and his colleagues found that a specific piece of a protein on Chlamydia's surface is identical to another one in the heart muscle of mice known as "myosin".

Their findings suggest that a Chlamydia infection, in the lungs or reproductive organs, for example, triggers a local immune response that can be followed by a wide-spread activation of the immune system.

Heart muscle targeted

An auto-immune disorder in the heart occurs when the host's immune cells also direct their attack towards the heart muscle myosin.

Previous studies have shown that injecting mice with this type of myosin resulted in inflammatory heart disease.

Heart muscle may become inflamed

Mr Bachmaier's group identified the specific section, or peptide sequence, of the myosin molecule that induced the disease.

They found matches in peptides from proteins on the outer membranes of three different Chlamydia strains.

When the group injected mice with the Chlamydia peptides, these peptides activated the same immune cells as the heart muscle myosin and also induced inflammatory heart disease.

Chlamydia infections are so common that most people can expect to experience at least one during their lifetime.

The researchers found that the genetic make-up of the mice on which they experimented determined whether they developed heart disease.

They believe the genetic make-up of humans may also make certain people vulnerable to heart disease triggered by Chlamydia.