[Microbiology] Contamination of 7H11 agar plates

martin rao via microbio%40net.bio.net (by martin.rao from gmail.com)
Wed Jul 16 01:33:39 EST 2008

Hello Laurn.

I came across your email on a blogspot by chance and thought that you might
be able to offer help with my problem. My lab work mainly involves
mycobacteriology and using 7H11 agar for plating is somewhat routine. Very
recently, I have been facing serious problems with contamination on unused
agar plates and I am rather perplexed as to how this could be. Of course,
contamination on agar is quite common a phenomenon but having said that, the
nature of the contamination seems puzzling. There happen to be grain-like
bacterial colonies seeded evenly within the agar, with a substantial number
of morphologically different colonies growing on the surface of the medium.
They are all white in colour, though the surface-growers have a
characteristic 'shiny, appearance. I am also told that this type of
bacterial colonies are also known as 'pressed-coin'. I haven't been all that
successful in deciphering this issue online and so, I wonder if you could
help me at all. Also, I must point out here that everyone in the lab where I
work uses the same autoclave machine. In addition, the supplementary
reagents that I add to molten agar prior to pouring plates are as well those
used for preparing liquid medium (7H9 broth) and I find not contamination
there, whatsoever.

I look forward to hearing from you.


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