I'm not sure if the cells are contaminated by staphylococcus (the round)? They look like the dead cells.
Dear Yan,
Assuming you're cultivating Hela cells or similar human cancer cell lines, the "clusters" look more like rounded-up, dead or dying cells. Bacterial cells are much smaller and proliferate much faster. After a day your medium would look cloudy. You can test it by transfering the floating cells/clusters in the medium to another well and wait for a day. If they don't proliferate, they are dead cells.
Good luck!
I agree with Christian. The round objects are definitely no bacteria since they are much too large. They look like dead cells to me.
Nevertheless, the cells do not look good. Some kind of contamination is possible, e.g. mycoplasms. Or the medium composition is not the right one for this cell line.
As others suggested above, they are certainly not bacteria.
Are these newly deefreezed cell line? or a line which you are using for the first time?.
If you have a bad defreeze, sometimes cells look like this and floating dead cells can be removed or replated for checking as suggested by Christian. We also had similar issues with a cell line after lentiviral transduction (mostly too much virus!). If you are new to this line, it is important to note that some cell lines grow with some amount of suspended cells.
If it is your familiar cell- line, already in culture and suddenly started to show this effect, its a matter to worry!! Good Luck.
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