In one of the largest studies to date examining the idea that germs may help cause heart disease, researchers said on Monday that heart patients who took antibiotics were less likely to suffer heart attacks or severe chest pain a year later.

But, to their surprise, the British researchers who conducted the study said the bacteria they thought might be responsible for heart disease did not seem to be the culprits.

Two different types of antibiotics worked — suggesting that an unknown microbe may be behind some of the cases of heart disease.