Have you tried subculturing these colony types to another type of
general laboratory medium, like TSA or TSA with 5% sheep blood to see
what they are? Have you tried staining these to see what they are? Are
they acid fast? I would think that 7H11 would grow regular bacteria, as
I do not believe that it has any antibiotics in it to inhibit non-acid
fast bacterial growth. If they are some sort of Bacillus sp. then your
medium is not being properly sterilized by your autoclave.
Have you run controls on your autoclave to make sure that it is getting
to the right temperature? You can buy commercial autoclave controls in
various formats. How long are you sterilizing for?
Are the plates you are pouring into sterile? Are you pouring into plates
that have had the package opened for quite some time?
Are you somehow introducing something nonsterile into your medium after
you autoclave but before you pour? You're not stirring the medium with
anything that is not sterile, are you? Do you cover your flask in the
autoclave? We used to put gauze over the top of the flask and seal with
autoclave tape. This let the steam out but kept stuff from falling into
the media while it cooled off in the 56 degree water bath until we
poured. Hopefully your flask is not open to the air in the time between
autoclaving and pouring.
If it is down in the agar then it is a sterilization problem. I've never
made 7H11 - always used commercially made plates - so don't know how
tricky it is to make. If your supplements don't produce contamination in
your 7H9 broth, then the only explanation is that your autoclave is not
functioning correctly and your media is not properly sterilized or you
are introducing a contaminant post-autoclaving.
Judy Dilworth, M.T. (ASCP)
"martin rao" <martin.rao from gmail.com> wrote in message
news:mailman.757.1216215400.3533.microbio from net.bio.net...
> Hello Laurn.
>> I came across your email on a blogspot by chance and thought that you
> be able to offer help with my problem. My lab work mainly involves
> mycobacteriology and using 7H11 agar for plating is somewhat routine.
> recently, I have been facing serious problems with contamination on
> agar plates and I am rather perplexed as to how this could be.