It has long been suggested that stress can trigger heart attacks, but so far no one has been able to decipher the underlying mechanisms. In recent years there has been an increasing amount of evidence to suggest that, amongst other things, infection may also play a role. Now, in a recently published study, scientists believe they have found evidence to support this idea. In particular, they discovered that a stress hormone can cause films of bacteria shrouding arterial plaques to fragment, liberating the plaque which can lead to strokes or heart attacks. The results can be found in the journal mBio.
Atherosclerosis is a disease that results from the build-up of plaque deposits inside the walls of arteries. These plaques, which are made up of a combination of fatty substances, cholesterol and calcium, can restrict the flow of blood and consequently may lead to heart failure, heart attack or stroke.
Various lines of evidence led to the belief that bacteria may infect these plaques, growing in dense communities known as biofilms which are held together by various molecules secreted by the microbes. In order to test this theory, a team of researchers headed by David Davies of Binghamton University examined arterial plaques from 15 patients. They discovered that all of them tested positive for the presence of bacterial genes and 5 of them were infected with 10 or more different species. Of particular interest was that 6 of them contained the known biofilm-forming species Pseudomonas aeruginosa.
Previous laboratory studies carried out by the same group demonstrated that P. aeruginosa biofilms disperse when exposed to free iron. Iron is known to be released into the blood by cells in response to raised levels of certain hormones such as the stress hormone norepinephrine (noradrenaline). To investigate possible links, the researchers grew P. aeruginosa in artificial arteries until biofilms developed. They then flushed these arteries with norepinephrine and sure enough, the biofilms fragmented.
The researchers suggest that the increased levels of iron in the blood as a result of norepinephrine could cause P. aeruginosa to secrete enzymes that are capable of breaking up the polymer matrix holding the biofilm together. This could then cause the once stable plaque to break off and enter the bloodstream, blocking the arteries.
Although the researchers acknowledge that much further research is needed, especially in animals and humans, Davies told Nature that the work “introduces a completely unexpected potential culprit” in how plaques may cause heart attacks.
While the research is certainly interesting, some experts warn that the amount of noradrenaline used in the experiment is far higher than would be present in the human body. However, they also accept the possibility that levels may be higher at the site of the plaque.
The researchers intend on continuing their work in mouse models and also hope to investigate whether the arteries of healthy people contain bacteria known to form biofilms.