A newborn baby died from the pseudomonas superbug at Altnagelvin Hospital in Londonderry in December last year and three more babies succumbed at the Royal Jubilee Maternity Unit in Belfast in January.
In February Professor Dame Sally Davies, chief medical officer and chief scientific adviser for England, issued guidance to special care units dealing with the infection.
The pseudomonas superbug is found widely in soil and stagnant water but does not usually cause illness in healthy people.
Dr Mark Evans, from the Health Protection Agency, said:
Dr Chris Burton, medical director of the North Bristol NHS Trust, said other babies at the unit had been tested and 12 were found to have pseudomonas bacteria on the skin.
The trust said pseudomonas bacteria has been found in the water supply in the neonatal intensive care unit.
A premature baby has died in hospital following an outbreak of the pseudomonas bacteria, it emerged today.
The baby, who has not been identified, passed away in August at the Southmead Hospital in Bristol.
Dr Chris Burton, medical director of the North Bristol NHS Trust, said: