Hi. My Mum Has A UTI And The Test Result

Hi. My mum has a UTI and the test result was infection with Citrobacter Koseri. She has a very low immmunity, UTI is a very often problem for her, she has had in the past a tumor at tyroid that was removed and after that she has got a polyallergy at drugs, she cannot take strong antibiotics. What is this bacteria? Is it dangerous? What can my mum do? Thank you.