[Drosophila] drosophila cultures dying

[The Hanings]

Hello.  My daughter is working with D. melanogaster
for the third year for a science fair project. 
Beginning about 4 to 6 weeks ago about half of the new
cultures we made die within a few days.  We are using
a commercial media from Conn Valley Biological Supply.
 The affected cultures have a tan colored, mucoid
substance on the surface of the media.  They also have
a terrible odor.  The media has mold inhibitors so we
suspect a bacterial contaminant.  We submitted
cultures for bacterial identification, but the results
are not in yet. Any thoughts on what might cause this
die off?
Ben Haning


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