Another suspected case of a baby being infected by Pseudomonas is under investigation by medical staff at the Royal Jubilee Hospital in Belfast.
The source of the outbreak, which has already claimed the lives of three infants, has still to be identified.
Laboratory tests have not confirmed another case but the victim is understood to be suffering from the same Pseudomonas symptoms.
Six other babies have traces of the bacterial infection on their skin. They are being closely monitored to make sure it does not enter their bloodstream.
A major deep clean of the maternity unit has been carried out, but staff have been unable to trace the source of the infection.
Taps were removed while sinks and pipes were checked amid fears it could be in the water system.
Northern Ireland health minister Edwin Poots is expected to get a new report tomorrow.
He said: "We can't suggest at this stage that's the cause of the problem, but it's certainly one of the areas being investigated."
The first baby died on 6 January, the second a week later and the third last Thursday night.
Pseudomonas aeruginosa can be found in soil and stagnant water. The children who died were born prematurely.
The disease can also pose a threat to elderly people with weak immune systems.
The infection can be treated with the right antibiotic, but the third baby who died failed to respond to the treatment.
There are usually fewer than 80 cases of it annually across Northern Ireland.
A helpline set up to support expectant mothers worried about giving birth at the neonatal unit has been extended due to demand.
All other maternity services and wards in the hospital are fully operational and working as normal.