A fourth baby is believed to have contracted the killer disease, Pseudomonas, at Belfast’s Royal Maternity Hospital.
Doctors are anxiously awaiting the results of laboratory tests after the deaths of three babies at the neo-natal unit.
The latest victim has displayed symptoms of the bacterial infection while six other babies are being monitored.
Northern Ireland Health Minister Edwin Poots is to issue an update on Tuesday on the hospital’s attempts to trace the source of the deadly virus.
A major deep clean of the maternity unit was carried out over the weekend but staff were unable to trace the source of the infection.
Taps were removed and sinks and pipes checked amid fears it could be in the water system.
Minister Poots has asked for a full report from hospital staff before he addresses the Northern Ireland Assembly at Stormont.
Three babies, all born prematurely, have died since January 6th from pseudomonas aeruginosa which can be found in soil and stagnant water.
The disease is also a threat to elderly people with weak immune systems.
Northern Ireland’s Department of Health confirmed that the number of babies who have bacteria on their skin is now six.
A statement said: “It is not causing active infection in these babies. The babies continue to receive the neonatal care they require.
“As a precautionary measure, babies’ skin may be screened again as the situation requires to see if they are carrying the bacteria.
“All necessary precautions are being taken to avoid spread of infection.
“The affected area in the unit will remain closed until it is absolutely certain it is a safe environment in which to manage premature babies.”
Plans are in place to move expectant mothers to other hospitals if necessary but the hospital is currently fully operational and working as normal.
The statement added: “Neonatal care in NI is provided within a network across the five trusts and all the organisations involved are co-operating fully on a daily basis to ensure that specialist neonatal care remains available for all infants who require this level of support.
“In line with well established practice, some mothers due to give birth or babies who require special neonatal care may be transferred to another unit. This will be on the basis of specialist clinical advice to ensure babies receive the most appropriate care.
“Providing infants with a high level of care and supporting concerned any parents remains a key priority.”