Three different types of mould caused the hospital infection linked to the deaths of a number of heart surgery patients in Edinburgh but the source has still not been found.
In a statement to the Scottish Parliament, Health Secretary Jeane Freeman confirmed that six patients had contracted rare fungal infections at the Royal Infirmary of Edinburgh (RIE).
Some have since died - but an exact number believed to be five or under has not been disclosed.
Ms Freeman said the infections had been caused by three different types of mould not normally found in the hospital environment.
These are: Lichtheimia corymbifera; Exophiala dermatitidis; and Aspergillus.
Four operating theatres were closed as a precaution to enable special decontamination procedures to take place, including deep-cleaning with hydrogen peroxide vapour.
Two have since reopened, said Ms Freeman, and the remaining two would resume operations "as soon as possible".
READ MORE: NHS Lothian cancels heart surgery after infection deaths
In a response to a question from Green MSP, Andy Wightman, Ms Freeman confirmed that that the source of the infection has not been identified.
"We continue to search for that," she said.
All patients whose planned surgeries had been cancelled as a result have been rescheduled for surgery either this week or next, she added.
It emerged on March 21 that NHS Lothian had written to 186 patients who had undergone aortic valve surgery at the RIE during the previous six months to notify them of a "very low" potential risk from mould infections, and the symptoms to look out for.
The action was taken after six patients who had undergone cardiothoracic surgery at the RIE between March 2017 and November 2018 went on to develop an "unusual and difficult to treat" infection in their heart valves, which in some cases proved fatal.
The health board stressed that there have been no further known cases since November 2018.
Addressing the Scottish Parliament, Ms Freeman said we "need to accept that not all healthcare-associated infections are preventable", but that she has instructed NHS Scotland's National Clinical Director, Professor Jason Leitch, to seek information and expertise internationally on whether any more can be done.
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It follows a spate of high-profile hospital fungal infection deaths involving the Queen Elizabeth University Hospital in Glasgow.
Prosecutors are investigating the deaths of a 10-year boy and woman, 73, who contracted infections linked to pigeon droppings, as well as the death of a 63-year-old woman from a rare mould infection normally associated with soil or rotten fruit and vegetables.
Ms Freeman said: "We need to ask, were these types of infection always existent but masked by more common things like MRSA?
"Could it be that as these types of infection are going down, others are emerging? We don't know."
The Health Secretary added that Scotland has the lowest prevalence of healthcare-associated infections compared to the rest of the UK and the Republic of Ireland.
Scotland also "compared favourably" to European nations including Finland, Spain, France, Portugal and Greece, she added.
Alex McMahon , NHS Lothian Infection Control Lead said: “Scheduled Heart Valve Replacement operations have resumed at the Royal Infirmary of Edinburgh (RIE) following the completion of a rigorous programme of precautionary specialist cleaning and testing.
“Patients whose procedures were postponed last week have been given new appointments and they and all other patients receiving this procedure can be assured enhanced infection prevention and control measures are in place to ensure their safety.
“NHS Lothian has been rigorous and proactive in its investigation of this matter. We recognise it is worrying for patients to receive a letter warning of infection risk – however low and would urge anyone who may still have concerns to contact the helpline if they have not already done so."
Tory health spokesperson Miles Briggs said: “My deepest sympathies are with patients who have been infected, the families and friends of the patients who have died from contracting this infection, and the 186 patients who will be a deeply distressed after being notified that they are at risk of infection.
“The infection of 6 patients, some of whom have died, once again raises serious concerns about the healthcare environment in Scottish hospitals.
“NHS Lothian were absolutely right to contact the 186 patients who were identified as at risk of being exposed to infection.
“I am pleased to hear that 26 patients have so far come forward, and I would encourage all other patients who have received a letter to come forward, so that they can be checked for infection.
“The top priority must now be reaching out to all patients who have been identified as at risk of infection and all measures must be taken to ensure they are infection free.
“Recent events across hospitals in Scotland have called into serious question how effective the Patient Safety Initiative is and what additional biological infection prevention can be developed in light of the cases we have seen across NHS Scotland.
“It is vital the SNP Ministers learn from recent infection outbreaks to review processes and make the necessary improvements, so that future risk can be mitigated.”