It is reported that the Western Health Trust dealt with three cases of the pseudomonas infection before last Christmas in Derry.
One baby being treated in Altnagelvin's neo-natal unit died, while another made a full recovery.
A third was transferred to the Royal Victoria hospital in Belfast.
All three infants had a different strain of the infection that is currently being investigated at the Royal's neo-natal unit.
It was confirmed this morning that there is another suspected case of a baby being infected by pseudomonas at the Belfast is hospital
The source of the outbreak has not yet been identified.
Northern Ireland's health minister is expected to make a statement on the matter later.