Patients in an intensive care unit in the Royal Victoria Hospital are still being screened for a rare infection described by the Chief Medical Officer as "difficult to treat and eradicate".
The infection is caused by a multi-resistant Acinetobacter baumannii bacteria (MRAB).
An intensive deep clean of the regional intensive care unit has been carried out with patients being monitored after the unusual multi-resistant micro-organism was identified by the Belfast Trust.
Four patients are believed to have been treated for the bug. However, there have been no new cases in the past 10 days.
It is understood two patients remained in isolation last night. The micro-organism is found on the skin and in the environment and is normally harmless, but can cause infection in patients that can be difficult to treat.
Brenda Creaney, Belfast Health Trust's director of nursing who also leads the trust's infection control policy, said the outbreak was discovered three weeks ago and was being "very well-managed" by the clinical and support staff.
In a statement the Belfast Trust said: "An outbreak has been declared and the unit has been undergoing an intensive deep clean process over the weekend in line with patient safety and clinical advice and all infection control procedures are being regularly monitored.
All patients remaining in the unit are being monitored in accordance with national guidelines.
"A small number of patients have tested positive for this infection related to this organism and are currently isolated.
The trust would like to assure the public that robust infection prevention and control measures are in place and we are working closely with the Public Health Agency and all other trusts to control the spread of this organism."
The trust has asked visitors to intensive care to help prevent the spread of infection by washing their hands when entering and leaving the unit.
Miguel A Valvano, Professor of Microbiology at the Infectious Diseases Centre for Infection and Immunity at Queen's University Belfast, explained MRAB infections can occur in the bloodstream, skin and in the lungs.
"These infections are particularly important in hospitalised patients with medical conditions that weaken their immune system, and cause prolonged hospital stays," he said.
"Patients that require artificial devices (catheters, ventilators, dialysis), and especially those who are seriously ill, have the highest risk."
Prof Valvano added: "The most common form of A. baumannii infection is pneumonia, which occurs in patients under mechanical ventilation in intensive care units. Unlike healthy individuals, severely ill ventilated patients cannot properly cough and clear bacteria from the respiratory tract, so that colonisation of the airways via environmental exposure can lead to infection of the lungs."
He said infections by this bug were of "great concern" for public health because of multi-drug antibiotic resistance.
Dr Michael McBride, Chief Medical Officer for Northern Ireland, said Acinetobacter can live on the skin and may survive in the environment for several days.
"It is difficult to treat and eradicate," he said. "Acinetobacter baumannii species found in hospitals are sometimes resistant to many antibiotics and the infections that they cause can therefore be difficult to treat.
"Transmission can be reduced by careful attention to infection control procedures such as hand hygiene, environmental cleaning and disposal of exposed medical equipment after patient discharge."
Dr McBride added that actions taken had included:
• Isolating all affected patients in side rooms.
• Identifying all contacts and screening all patients.
• Increased cleaning and reduced visiting times and visitors.