I've an old degree in micro but that was long ago; later worked in research labs at a good university, but that is long ago too now, and not with cell lines as such, so I'm no help. Do you have any nearby good university? thinking biomed library, access to useful journals or perhaps experts about contamination, and so on.
Never mind re my help, but maybe others here will have a better clue than I do either re your exact problem or who to ask.
Edit to say maybe there are online good biomed libraries I don't know about since back in my day, there was no internet.