[nat]

Dear all!
About one month ago i've noticed some dots in my ES cell culture. They seemed not to grow fast over time and don't move, but if I leave my cultures longer in the same flask I start to see more of them. The media doesn't change color but cells start to differentiate more. I cleaned incubator, hood and waterbath, also made new media from new reagents. So, after trashing cells and doing all this I thaw new vials of very early passages that I established 2 years ago and i start to see this stupid creature again!!!!! And also newly derived ES cells groing without serum(completely different media) start to have this bustards!
I tried Normocin and Plasmocin apart of Pen/Strep, doesn't help. Also I used fungi and yeast stain, they are negative. They seem to be too small as well. Bacteria???? Although I put some supernatant to the LB plate and nothing is growing for 2 weeks already!
I attach some pictures taken at 63x. They look round sometimes having a satellite smaller cell attached.  1.jpg (976bytes)
Number of downloads: 54  2_63x.jpg (904bytes)
Number of downloads: 46  5_63x.jpg (936bytes)
Number of downloads: 43
Please help me because I don't know what to do with them. I cannot simply continue my project because of this problem

[blackadder]

You could test for mycoplasma. Since you dont have a reference for your normal ES cells in the picture its very difficult to tell whether these are not just single Embryonic Cells.