Re: Tryptone broth test for sterility check

msalguod@xxxxxxxxx wrote:
We've started testing our microbiology dilution water for sterility
(after autoclaving) by adding some double strength tryptone broth and
incubating for 24 hours. I've never done this before.

What exactly constitutes growth?

Our first test had alot of what looked like white sediment on the
bottom of the sample and the smple was cloudy. I wasn't sure if this
was growth or some sort of precipitation.
I set another batch and I purposely contaminated some and put in some
plain DI water. Still they all looked the same.

I checked the autoclave settings and saw that they had it set for the
mininum 15 minute hold at 122° C.

I tried another batch with a 30 minute hold. The contaminated samples
still looked the same but the uncontaminated samples were only a little
cloudy. It made no difference whether the bottles were in the middle of
a cluster with no air space between them or if they were alone with
plenty of surface exposure.

Does the cloudiness constitute growth (I think it does)?

Do you find that sterilization of water requires more than a 30 minute
hold time?

Thanks.

it depends on volume

I would think 30 min is good for about 500ml

to be safe you can cook for longer. Most media don't mind a little extra time in the heat.

BTW there are formulae to tell you what volumes require what amount of time at what temperatures... take a look around.
.

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