Stroke study finds mouth bacteria in brain clots

Ensuring good oral hygiene could help to prevent stroke. This was what scientists proposed after finding DNA traces of oral bacteria in samples of blood clots that had caused strokes.

Researchers from Tampere University in Finland analyzed clot samples from 75 people who received emergency treatment for ischemic stroke when they attended Tampere University Hospital's Acute Stroke Unit.

The patients had undergone thrombectomies. These procedures remove blood clots by means of catheters conducted through arteries. The catheters can deploy stent retrievers and aspirators to reduce or remove the clot.

When they analyzed blood clots sampled in this way, the researchers found that 79% of them bore DNA from common oral bacteria. Most of the bacteria were of the Streptococcus mitis type, which belong to a group that scientists call viridans streptococci.

The levels of the oral bacteria were much higher in the blood clot samples than they were in other samples that surgeons took from the same patients.

Larger picture implicates bacteria in clots

The study forms part of a large investigation that Tampere University has been conducting for around 10 years on the role of bacteria in cardiovascular diseases.

This investigation has already found that blood clots that have caused heart attacks, brain aneurysms, and thromboses in leg veins and arteries, contain oral bacteria, particularly viridans streptococci. It has also shown that these bacteria can cause infective endocarditis, a type of heart infection.

The researchers believe that the new study is the first to implicate viridans streptococci in acute ischemic stroke.

A stroke is when the brain suddenly experiences a disruption to its blood supply. This starves cells of essential oxygen and nutrients and can result in tissue damage and loss of function in the brain.

The most common type of stroke is an ischemic stroke, which occurs when a blood clot reduces the blood supply in an artery that feeds the brain.

According to figures from the World Stroke Organization, around 1 in 6 people worldwide will likely experience a stroke in their lifetime.

One of the leading causes of stroke is a condition called atherosclerosis in which plaques form in the walls of arteries and cause them to narrow and harden over time. The plaques are deposits of cellular waste, fat, cholesterol, and other materials.

Depending on where the plaques form, atherosclerosis can raise the risk of heart disease, angina, carotid artery disease, and peripheral artery disease.

However, plaques can also lose bits into the bloodstream, or attract clots. If such an event affects an artery supplying blood to the brain, it can trigger an ischemic stroke.

Oral bacteria: Cause of stroke or 'bystander?'

In discussing the implication of the results, the authors note that streptococci bacteria from the mouth can cause serious infection, such as of the heart valves, when they enter the circulation.

There is also evidence that bacteria can activate blood platelets directly. Could this be a possible route to increasing stroke risk?

"Activated platelets" trigger cells that promote atherosclerosis and "speed up the development of atherothrombotic lesions," they write.

"Bacterial surface proteins of S. mitis," they add, "can directly bind to various platelet receptors."

In regard to the recent findings, the researchers note that while they show that oral bacteria are involved, it is still unclear whether they cause strokes or whether "their role is solely as bystander."

In the meantime, they suggest that: "Regular dental care should be emphasized in the primary prevention of acute ischemic stroke."