These are nasal epithelial cells that have been in culture for several weeks. We have grown 20 different samples of the same material using the same protocol with no such issues. Hoping this is an isolated case.
I also had a similar infection problem using 6- well plates. When I checked my unused plates I saw that some of them had hair like scratches on the inside I discarded these and did not have the problem since. It may not be what caused your problem but it is something to look out for. I hope this helps.