From the HBD Archive
Subject: Streak plates & purity
Date: 1992-03-16 22:39:53 GMT

I've been noticing alot of people talking about plating out Red Star
Ale Yeast to determine its purity. The most recent posting mentioned having
found 3 out of 1000 colonies being bacteria and many of the yeast colonies
being slow growing. This lead the person to not worry about contamination of
the yeast. The problem here is threefold. First, the media used is
presumably selective for yeast. This means that there may be alot of bacteria
in the yeast that simply won't grow well on it. Second, bacteria as a rule
grow much faster than fungi (including yeast) That means that 3 bacterial
cells stand a fair chance at overrunning 997 yeasts. Third, are the slow
growing yeasts. the chance that they are mutants of the correct strain is
low, otherwise they would be found to some extent on the plates streaked with
the strong colonies. The mutation rate is not high enough to produce that
number of mutants in a supposedly pure culture. This doesn't worry me as much
as the bacteria though because the fast growing yeast should overrun the slow
growers. I would be interested to see the results if someone were to plate
the yeast on media selective for bacteria such as TSA or CBA. Brucella agar
incubated anaerobically would also be interesting. By the way, I'm just
putting forth my opinion and suggesting an experiment, so please nobody
mistake this for some kind flame. Sorry if I sound paranoid.

Mike Valentine

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