The parents of a baby who died in hospital from the Pseudomonas bacterium have called for a public inquiry into the outbreak.
Caoimhe Campbell and Gavin Burke's son Caolan was 10 days old when he died in Altnagelvin Hospital in Derry.
It happened in December, weeks before three babies died from a different strain of the bacteria in Belfast.
The couple say they want a public inquiry.
While they acknowledge their son, who was born 15 weeks prematurely, was very seriously ill, they said he was showing signs of improving, before his death on 14 December.
The couple were told their child had died of Pseudomonas by doctors who visited their home after Caolan 's death.
In a letter dated 4 January, two consultants from Altnagelvin's paediatric department expressed sympathy to the couple on the loss of baby Caolan.
Shortly after Caolan's death, another baby who had also tested positive for the bacterial infection was transferred from Altnagelvin to the Royal Jubilee Maternity.
While that child had a different strain of Pseudomonas, according to a letter sent by Chief Medical Officer Micahel McBride on 22 December, at no stage was the Belfast Health Trust advised that a child had died at Altnagelvin.
Mr Burke said the two hospitals should have communicated better.