Four children are now receiving treatment for infections which may be linked to the water supply at the RHC in Glasgow.
Health Secretary Shona Robison has apologised to patients and families affected.
NHSGGC said it is working with experts from Health Protection Scotland, Health Facilities Scotland and Scottish Water.
An NHSGGC spokesman said: "These bacteria can pose a risk to patients whose immunity is compromised, however we have put in place robust infection control measures to protect our patients.
"We have now extended this testing and infection control measures to four wards treating the most immunity compromised patients in the Queen Elizabeth University Hospital to ensure that we take every precaution."
The spokesman added: "Three children continue to receive treatments for infections that may be linked to bacteria found in the water supply.
"There are no reports of any patients being infected by bacteria from water in the adult wards."
A series of infection control measures were introduced to the affected RHC wards at the weekend and water filters are to be installed.
He added: "The news of contamination of the water supply in the cancer ward at the children’s hospital in Glasgow has caused understandable worry and concern for parents of very sick children."
"I have been assured by the health board that they have indeed been keeping parents informed.