The use of an inhibitory should solve the problem. Use of IMA (inhibitory Mold
Agar) that contains chloramphenicol inhiibits bacteria while fungi grow well.
You could also use BHI agar with cycloheximide.
Filamentous fungi are everywhere! If you are into scaring yourself, try a
sample from the air conditioning ducts at work or home. Soil and compost are
also great sources. Also, that unidentifed stuff in the back of the