A neonatal intensive care unit was being cleared yesterday after three babies died from the same infection in two weeks.
The outbreak at the Royal Maternity Hospital in Belfast has been traced to a bacterium called pseudomonas aeruginosa, which can affect the lungs, skin or gut.
Expectant mothers booked into the maternity hospital with a high risk of giving birth prematurely are to be transferred to other hospitals in Northern Ireland, the Irish Republic or Britain.
Colm Donaghy, chief executive of the Belfast Health and Social Care Trust, said the "first priority" was the safety of the babies and ensuring "the work that we do keeps babies safe". A full investigation was being carried out, he said.
Consultant neonatologist Clifford Mayes, who works in the unit, said the first death had occurred on January 6 and the second on January 13, but doctors did not suspect there was a problem until Monday night when results of tests showed both had died of pseudomonas infection. The third death occurred on Thursday night.
Pseudomonas can normally be treated with antibiotics but doctors were unable to save the third baby.
The 31-cot unit has 550 admissions per year. Its patients are extremely premature, very small babies who are highlly vulnerable. All those in the unit at the time have been swabbed for signs of the infection and the unit is being cleaned.
"We've spent the last couple of days trying to support the parents," Dr Mayes said. "Every baby has been screened with skin swabs, looking for any evidence of pseudomonas. It is a germ which can be on your skin and not cause any harm but with a sick patient it can cause very serious problems."
"In itself it is not infectious, but because it exists in water or where things are moist, what we are having to do is investigate the unit very thoroughly to try and identify a source."
The mother of a baby girl in the unit was anxiously waiting for the results.
"She's just seven weeks old and was born at just 24 weeks, weighing one pound seven ounces," she said.
"She has come so far, past illness and the fact that she could get sick again – we really don't want to think about it."
Health Minister Edwin Poots said the authorities were taking the matter very seriously. "It is important we remain calm. Infection control teams are now trying to identify the source of the infection and minimise the risk of spread to other babies in the unit," he said.
A helpline has been set up for parents on 028 90 635 389 and admissions are being restricted.
Pseudomonas: The deadly threat
The pseudomonas bug is not infectious but can be spread by touch – commonly on hands or medical equipment such as catheters and feeding tubes, causing cross-infections in hospitals and clinics.
Its tough nature and the severity of the symptoms it causes mean it is a priority for infection control departments.
There have been a number of outbreaks in hospitals in the UK, with the source sometimes traced back to contaminated water in sinks and drains.
It can survive for several days on surfaces,increasing the risk of it being passed on to patients.