differential media

Fred & Rachel Brusseau rbruss at ix.netcom.com
Sat Mar 7 08:35:01 EST 1998

Basic Micro questions:

My goal is to identify coliforms, beta hemolytic strep, and pathogenic
staphylococcus from human milk. (I don't think I'll find them, but I
need to run a test to make sure they aren't' there.)  I've plated the
milk on MacConkey, 5% blood, and Mannitol salt.

Does anyone know where I can find a picture of a coliform growing on
MacConkey? (preferably on the net, but books are OK). I've looked in my
micro text, Berkey's, and a micro color atlas. According to Difco, it's
supposed to form brick red colonies. I have purple clumps and bumps,
(don't really look like colonies) at 24 hours incubation time. It might
just be the dye.

Can anyone confirm that Mannitol will turn yellow with pathogenic
staphylococcus, but stay orange with EVERYTHING else?

Is it true that ONLY beta hemolytic strep and pathogenic staphylococcus
will form a clear ring on blood. If I get this ring, can I just do a
gram stain of the colony to see of it is a staphylococcus (clumped
cocci) or strep (chain cocci)?

You can email me, or reply to posting
Rachel Brusseau

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