The infections had spread into the bodies of two of the babies at the Edinburgh Royal Infirmary, while the other two babies only showed the bacteria on their skin, the Scotsman newspaper reported.

The cases, from April and May, followed two previous "clusters" that had not been identified until Tuesday, the newspaper said.

Dr. Alison McCallum, director of public health for the hospital's parent company, said all the clusters were dealt with by the unit and the problem appeared to be over.

She said the outbreaks prompted a "deep clean" of the unit, which admits around 700 babies a year.

McCallum said all babies are now routinely screened for MRSA on admission, and screened again for it weekly.

MRSA, or Methicillin-resistant Staphylococcus aureus, is a strain of bacteria that is resistant to all penicillin. It was discovered in Britain in 1961 and is now widespread, particularly in the hospital setting, where it is commonly referred to as a "superbug."