Chlamydia and clogged arteries.

Physicians usually attribute coronary artery disease to four classic risk factors: advancing age, cigarette smoking, hypertension and high blood cholesterol. Now, Finnish researchers think they may have stumbled across another factor: Chlamydia pneumoniae, a bacterium commonly held responsible for respiratory infections such as pneumonia and bronchitis.

Pekka Saikku of the University of Helsinki, who led the Finnish group, warns against confusing this microbe with its sexually transmitted cousin, Chlamydia trachomatis. He and his colleagues describe their findings in the Feb. 15 ANNALS OF INTERNAL MEDICINE.

While conducting a large, long-term study of a new cholesterol-lowering drug, Saikku's team discovered that men showing evidence of a chronic C. pneumoniae infection -- signaled by antibodies and bacterial remnants called lipopolysaccharides in their blood -- were nearly two times more likely to have a heart attack than men who lacked such evidence.

To explain how the bacteria might promote coronary artery disease, Saikku theorizes that white blood cells called macrophages inadvertently carry the lipopolysaccharides into the bloodstream after fighting C. pneumoniae in the lung. Once in the blood, lipopolysaccharides bind with cholesterol, making it toxic to the nedothelial cells lining the arteries. In addition, they indirectly block a crucial fat-destroying enzyme, allowing fatty deposits to accumulate on arterial walls.

While C. pneumoniae becomes a new risk suspect for coronary artery disease, the researchers have yet to catch it in the act. Verification of their theory would be a significant finding, says Saikku, because C. pneumoniae infections are easily treatable with antibiotics.