I've been growing Saccharomyces cerevisiae (strain BJ2168) in shake flask for protein expression. After 72 hours' culture time I harvested the cells, but found a layer of suspicious-looking greenish goo on the edge of the pellet. After I resuspended the pellet in lysis buffer, I noticed that the the suspension also showed a trace of greenish color. I don't know if it is normal or is due to some bacterial contamination--I did have pen-strep added into the culture, so I'm really frustrated about the unknown green stuff. Anyone have had similar experience? Thanks!