Dear Cynthia, 

That particular contaminant doesn't sound familiar to me.  However, I have found that the most sure-fire way to get rid of contamination of any kind is to grow the cells on bacterial lawns.  Make a lawn of Klebsiella or other suitable food bacteria on a 10 cm SM plate and deposit a few contaminated cells at one edge of the plate.  Let the Dicty eat their way across the plate (this may take a week or so), and once fruiting bodies are formed, pick spores from the opposite edge of the plate from where you deposited the cells initially, taking care not to touch the surface of the agar.  You may have to do this a couple of times for a particularly aggressive contaminant (for example, filamentous fungi), but the Dicty will eventually out-eat and out-migrate every contaminant I have ever encountered.  If your contaminant still re-appears, then you can be almost certain that it is being introduced during media preparation or subculturing.  

Good luck!
Dan Dickinson


On Nov 8, 2013, at 10:32 AM, "Damer, Cynthia Kay" <[log in to unmask]> wrote:

> Dear Colleagues,
> 
> My lab is currently experiencing a culture contamination problem that I have not experienced previously.  We are culturing Dicty in HL-5 supplemented with penicillin and streptomycin on plastic sterile culture dishes.  The contaminant appears to be very small (less than a micron) and spherical and is not able to move.  In addition, it is very slow growing.  However, as the contaminant becomes more numerous, the Dicty cells become rounded and eventually die.  Unfortunately, even when we thaw out strains in new media, the contaminate appears again.  We've also tried centrifuging and washing many times and greatly diluting the cultures.  This seems to get rid of most of it, but it eventually grows up again.
> 
> Any advice would be appreciated.
> 
> Thanks,
> 
> Cynthia
> 
> Cynthia K. Damer
> Professor
> Biology Department
> Brooks Hall Room 229
> Central Michigan University
> Mount Pleasant, MI 48859
> (989) 774-3455