Out of the last batch of plates I did, all but two developed mould colonies within a couple of days - including one of the negative controls. I've never had a problem like this before, so I've been racking my brain to figure out what the problem could have been... Streaking technique was immediately ruled out by the fact that the problem also occurred on a plate that hadn't been opened since I poured it. Contamination when pouring is always a possibility, but like I say, I've never had a problem before, and certainly not of this magnitude, and if anything, my pouring technique has been getting better... Then I remembered: when I loaded the (glass) Petri dishes into the pressure cooker for sterilisation, I'd loaded them in two vertical stacks rather than edge-on (which is what I usually do), and the autoclave tape had only changed colour on the top one on each stack. At the time, I'd figured "Hey, they've been in there for 25 minutes, they'll be fine..." Well, you would, wouldn't you? Most people figure a couple of minutes boiling is adequate, so 25 minutes in the pressure cooker has to do the job, right?
Now, I didn't keep perfect track of which plates were on top of the stacks and which subsequently developed mould, but I do know that one of the clean plates was from the top of a stack, and I think the fact that only two of the plates didn't develop mould, combined with the fact that I've never had a problem like this before, is jolly suggestive...
I don't really like using disposable labware (I hate chucking stuff out after a single use) but I was seriously considering changing to single-use disposable Petri dishes anyway, simply because cleaning and sterilising glass ones is such a pain in the arse... And I'm leaning even more heavily in that direction now.
Moral of the story: "good enough" isn't good enough, and autoclave tape isn't just for laughs. If the tape doesn't change colour, it's not sterile, and if it's not sterile, it's contaminated.
(Thought about putting this in "Cleaning etc", but it's probably more at home here. For most brewing activities, "good enough" probably is good enough. Plates are very unforgiving... And that's the point of doing them.
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