small dots in cell culture - (Aug/04/2012 )

Hi everyone
I work on PC-3 and NIH-3T3 cell lines. It is for 2 weeks that I have small dots in my culture that slow the rate of cell growth. they appear after the first passage of defreezed cells but in free culture medium they don't appear. Thses dots result in cell death and collapse. has every body any helpful idea?
thanks

There are several threads in bioforum discussed black dots in cell culture. You can take a look to see which yours look like.
http://www.protocol-online.org/biology-forums/posts/35069.html
http://www.protocol-online.org/biology-forums/posts/12608.html
http://www.protocol-online.org/biology-forums-2/posts/21330.html
http://www.protocol-online.org/forums/topic/19403-black-dots-in-cell-culture/
http://www.protocol-online.org/forums/topic/12942-black-dots-in-cell-culture
http://www.protocol-online.org/forums/topic/8829-is-this-contamination-or-otherwise
http://www.protocol-online.org/forums/topic/19925-contamination-in-cell-line

Could be bacteria or yeasts - put them under a high power microscope, yeasts appear in the classic budding shape of ovals with smaller ovals coming off the sides. Bacteria will usually appear as a haze or shimmering background.

Read the thread on black dots in cell culture, which is pinned at the top of the cell culture subforum. Like pcrman suggested while I was typing