To determine if there is any contamination, you can apply approximately 100 ul of FBS onto LB agar that does not contain antibiotics. Then, allow it to incubate at 37 degrees overnight. If no growth is observed, it indicates that there is no contamination.

Hello Inês S. Pinto

It is not contamination as the cells in the pic seems to be quite healthy. I feel it is debris from FBS. At times, thawed serum will appear cloudy. It is usually the result of normal serum components. Also, fibrin appears in small quantities as fine flake or may occur as a lump of a semi-solid substance. The presence of this material does not alter the serum’s performance and may be mistaken for microbial contamination. In such a case, you may allow the serum to stand. The sediments will eventually fall to the bottom of the bottle and the serum can be decanted.

Best.

Hello Inês S. Pinto,

I fully support Malcolm Nobre's observation. What you're seeing isn't contamination; it's likely debris from the FBS (Fetal Bovine Serum). Following Nobre's suggestion, allow the serum to stand and avoid using the bottom 5 ml, where most of the debris tends to accumulate. Alternatively, you can filter the serum before adding it to your media.

Best,

Anjali