[Microbiology] Re: Contamination of 7H11 agar plates

Maureen Boost [HTI] via microbio%40net.bio.net (by htmboost from inet.polyu.edu.hk)
Fri Jul 18 02:40:03 EST 2008

We have also have the insect problem -but that led to "trails" of colonies rather than evenly seeded throughout .
This sounds more like something in the agar - if other people using the same autoclave have no problem  - 
are you filtering your additives with .22 filters and have the filter holders been properly sterilised? 
Otherwise are you using a dispenser to fill the petri dishes - check that tubing is regularly sterilised.


>>> <tmccloud57 from sprintpcs.com> 18/07/2008 4:33 AM >>>

       A plant pathologist I know, who brings plants into the lab and
cultures microbes from them, had a long-unexplained, un-inoculated
Petri dish contamination problem until he started storing plates in
deep, sterilized plastic tubs.  Turns out mites, so small you don't
normally see them, were crawling around, carrying microbes into
everything.   Choose tubs big enough so mites can't climb into them.
Good luck,  Tom McCloud   

On Wed, 16 Jul 2008 14:33:39 +0800, "martin rao"
<martin.rao from gmail.com> wrote:

>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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