Help!!! My screening is contaminated!

katja ludin katja.ludin at
Mon Nov 15 06:57:36 EST 1999

Why don't you set up a new screening right away, the best with a newly
streaked out starter Y190. In the mean time do a beta-galactosidase
assay with some plates of your contaminated onces now and in four days
(I guess you don't have a positive control along with your screen. It
was handy in my screenings always to have something that will show the
blue color). If you cannot see any hits in a representive amount of
plates, say 10 %) don't follow up the contaminant plates, do it right
from the start. It saves time by the end.
For once, it's probably not E.c. growing on your plates, the
minimalmedia is too poor for those fellows.

Schmidt.Thorsten at wrote:
> Hi,
> I have a serious problem and hope so that someone of you
> could help me:
> Almost all of my plates (80 !) of my yeast two hybrid
> screening (GAL 4 System, Y190) seem to be contaminated with bacteria!
> The plates are now incubating for 6 days and several yeast cultures
> (approx. 0.5 mm diameter)have formed all over the plates, as expected.
> But all over the plates (in the area around the colonies)
> a light "smear" is visible.
> I streaked over a part of some plates and streaked the cells
> on a slide.
> Under the microscope many healthy yeast are visible as well
> as some yeast with clear bacteria IN it (they are moving in the yeasts
> cells) as well as single bacteria cells and some clusters of bacteria
> forming around lysed yeast cells.
> After that,I picked single yeast colonies, they are looking healthy and
> hardly unaffected.
> What can I do now?
> Do I have to throw all these 80 (!) plates away and start a new
> screening?
> Or could I still use them?
> Will the bacteria seriously affected the results of the screening?
> Could I somehow "spray" any antibiotic on the plates?
> Normally, I would like to incubate the plates for another four days
> before performing the beta-Galactosidase-Assay.
> Should I wait these days or should I proceed as fast as possible?
> I would imagine that the E.coli might have overtaken the
> complete plates and killed all yeasts if I will wait longer.
> Do you have experiences with that?
> Thank you so much in advance for your answer!
> Everything would help!
> Thorsten
> Sent via
> Before you buy.

Katja Ludin
Seftingenstrasse 25
3007 Berne 
katja.ludin at

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