[Yeast] contaminations in a two hybrid system

[Barbara Tomassini]

Dear list,
I am trying to have some positive clones in a yeast two hybrid system 
and now I am observing the growth of some colonies that seems to be 
S. Cerevisiae colonies. They form colonies on agar plates in SD 
minimal TLHA medium but they seems to be worms at the microscope! 
Have you ever had such clones and do you know which kind of organism 
is? How can I avoid this contamination?
Thanks!
Barbara