Hello, Oscar.

It is the FBS that prevents further progress of infection from the already-contaminated cells. Fetal serum has many unknown factors, even microbe-killing activity. The contaminated cells with low titer of microbes would not grow in KSFM due to the lack of beneficial effects from FBS even in the presence of the antibiotics. While the cells in FBS grow faster due to the presence of various growth factors and repressing microbe proliferation due to microbe-killing activities along with the help of the antibiotics added. We all understand the limitation of antibiotics supplements in cell culture medium. See the link. https://www.researchgate.net/post/Why_do_the_researchers_use_fetal_bovine_serum_or_calf_serum_instead_of_using_bovine_serum_during_an_experiment

So, check your cell stock for extensive microbial contamination first.

Best wishes.

Hello Oscar, 

I do not have clear answer to your question, however i would like to say that we have had the exact same problem in our lab during the past year. We are culturing primary nasal epithelial cells in KSFM supplemented with BPE and EGF and are having reoccurring infection problems that we have never experienced in the past. Have you successfully cultured your cells in this media before you started having problems?   

Thank you Sang, it seems checking the cell line for possible contamination might be the best idea and I will keep an eye out for FBS effects on it.

Cecilia we actually culture cells in FAD medium (3 parts DMED + 1 part F12 and supplements, can send you the whole recipe if you'd like) with FBS. We only have problems when during certain experiments we switch to KSFM where suddenly infection becomes much more prevalent (this has been a recurring problem in the lab apparently), so indeed it seems that FBS does help prevent infection effectively. Hope this helps.