Though SIDS is the third leading cause of death among babies before their first birthday, its cause or causes remain a mystery.

The study, published in the Lancet, from tests on more than 500 babies found high levels of Staphylococcus aureus and Escherichia coli in babies who died for unexplained reasons.

One explanation could be that the bacteria release deadly toxins, which damage the young heart, lungs or nervous system.

But bacterial growth may also be a secondary effect of other known risk factors like over-heating, parental smoking and lying a child on its stomach.

"As yet, we do not understand the true significance of the findings," added Dr. Nigel Klein, co-author of the study and professor of infectious disease and immunology and head of the department of infection at the University of London and Great Ormond Street Hospital for Children in the United Kingdom. "At present, a causal link has not been established. As such, there are no direct clinical implications."

"The next step is not just to look for the bacteria in these babies, but to look for the toxins that the bacteria are producing," SIDS researcher James A. Morris, M.D., of the Royal Infirmary Lancaster, says. "That is where this new science comes in."

Every year about 250 infants die from SIDS. The best way to reduce the risk is to ensure that babies sleep on their backs and that their parents do not smoke, the Foundation for the Study of Infant Deaths recommends.