Unknown contaminant in S. cerevisiae culture - (Jun/27/2012 )

The problem I´m dealing with is a contamination in my S. cerevisiae cultures (GG 595, W303, BY4741) of unknown nature which suddenly occurred in our lab some months ago. It looks like G+ motile cocci swimming in the culture. All of our attempts to isolate them (by straining or in semi-liquid media) have been unsuccessful. They´re also resistant to ampicilin, kanamycin and chloramphenicol. Yeast cells in the presence of contaminants shrivel up and don´t grow well (especially, when used for transformation - LiAc method).

Do you have any suggestions? Which species this could be and how to identify them?
Thanks a lot.

you should be able to get rid off them by streaking the culture on media without antibiotics. I would suggest at least 4 times subculturing. Make sure that your culture media are not contaminated and that you pick one single colony as soon as you see them, grow them, and check for purity.

Otherwise get a new batch of yests to grow - usually your lab should have a uncontaminated stock somewhere in the freezer.