We are usually able to pour NGM plates (using a Wheaton pump) on the lab bench, but we have a contamination problem that has reared its ugly head again. I was wondering if anyone else had problems with a similar contamination.
The contamination is small white spots that appears throught the NGM in the plates. It is relatively slow growing as it does not usually appear on the plates for at least a week (and sometimes longer.) It does not grow very well at all at 4C (where we store our plates long term) - often we will not be able to detect the contamination until the plates have been at room temperature for a few days (after cold storage) at which time the contamination takes over the plates with multiple spots. (They almost look like small yeast colonies to me.) We autoclave our batches of media and the tubing for the pump. (Phosphate buffer is added after autoclaving the media.)
Any suggestions for combatting this problem would be most welcome.
Thank you for the great tips. Some of the suggestions are things that we have tried in the past and others, we will be sure to incorporate into our protocol for the future. Our colleague who works with pathogenic yeast strains took a sample from our contaminated plates and determined that we have a staph, not yeast, problem. I suspect that the problem is with one of the solutions that is added after autoclaving so we are remaking the solutions (in much smaller aliquots) and trying again.