Eighteen children are involved in a contamination scare at Our Lady's Children's Hospital in Crumlin in Dublin.
The hospital insists there is no immediate impact on children's health.
The contamination problem involves children who underwent a colonoscopy with a contaminated colonoscope between 17 May and 5 July.
The problem emerged when a crack was found in the colonoscope and tests on 6 July last showed the crack contained a bug that can cause infection and cannot be cleared by antibiotics, but can be treated.
The bugs concerned are ESBLs - extended spectrum beta lactamase - which often live harmlessly in the bowel.
But the bug can cause problems in patients receiving intensive care or chemotherapy and in other areas.
In a statement, the hospital said it first started contacting parents today in relation to the contamination alert.
It said that 15 of the 18 families concerned have been successfully contacted.
The remaining three have not been contacted, as they are on holidays or have not answered their phone.
The hospital said that follow-up letters, an information pack and a sample kit will be issued to each family.
It said when the problem emerged on 6 July, patient tracing started to do patient screening.
The hospital microbiologist identified on 10 July that the problem affected only one colonoscope and that the microbiological growth involved the bacteria, ESBL.
It said it had to undertake a careful look back to ensure the correct patient group was identified and insisted there were no health issues arising from the time lag in contacting patients.
The hospital said that a positive screen has no immediate impact for patients, but it may affect the first line antibiotics chosen to treat a serious infection such as sepsis.
All scopes are examined and cleaned every four weeks.
The scope involved was cleaned and in use since May.
Antibiotics can treat ESBL infection, but they will not clear it from the bowel and antibiotics can encourage ESBLs to grow in the bowel again.
The hospital has apologised for any distress caused by the contamination alert and said the results of tests will be notified to families immediately.
It also said there will be a short-term impact on access to colonoscopies as a result of the incident.