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I have been tissue culturing several species lately, and each one of them has the usual problem with contamination associated with all tissue cultures. The majority of these are removed by sectioning, transferring, and repeating. However, I seem to always have a similar looking contaminant in every culture that I am unable to identify. The little black dots appear at the end of the hyphae, and regardless of how often I transfer them away, I seem to keep bringing this with me. To the plain eye it looks like someone shook black pepper on the mycelium in the petri dish. I have included a picture at 75x with this post, and would love to hear from other tissue culturers as to what this is. It is possible it is not a contaminant at all, but the way the mycelium concentrates it more towards the edge of the petri dish as it expands makes me think contamination.
In that picture it looks to be laying flat. If it's standing up like a dandilion in your front yard, I'd say aspergillus. It has both green and black presentations. It's usually not smooth like that though.
One way I've found to beat those pesky contaminants is through agar pasteurization. Make up a fresh batch of agar. When it has cooled enough to pour into the dishes(+/- 120F-130F or 49C-55C), pour a layer right over the top of the dish with both healthy mycelium and the contaminants. Be sure to cover all the growth in the dish with a layer of hot agar. The mushroom mycelium, being a coherent network will usually survive the assault, but the contaminants will be neutralized for a few days. Watch the dish carefully. Usually within 48 to 60 hours, the mushroom mycelium will make its first appearance through the fresh layer of agar. Use an inoculating loop rather than your scalpel and immediately scrape a tiny bit of this mycelium off the surface of the top layer without taking any of the agar itself. This usually results in a very clean transfer of mushroom mycelium before the contaminant has had a chance to recover. RR
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