I've experienced some problems culturing human cancer cell lines cultures and I can't really be sure if it is some kind of contamination. My main concern is that I observe black "dots/spots" in the cultures, mostly adhered to the flasks. Initially I thought that it could be some cellular debris from non-resistant cells (I use G418 to select clones) or FBS precipitates. As this pattern persists throughout subcultering, I'm considering that it could be a contamination. These cells grow in the presence of 2 antibiotics (Pen/Strep), and wondering if they are camouflaging a contamination I cultured cells in the absence of antibiotics. The pattern of the "black dots" are pretty much the same in cells cultured with or without antibiotics; some with rounded form, others looking like small wires, but mostly with an irregular pattern. Also, the media does not change color along the time. In all the cases, they do not move from one place to another, actually. I am attaching some images…Do you think it’s a sign of some kind of contamination?