You wrote:
> To: microbiology at net.bio.net> From: Angela.B.Lee at dartmouth.edu (Angela B. Lee)
> Subject: EMB, aargh!
> Date: 22 Nov 1994 07:37:26 GMT
>> I have a couple of EMB plates that are giving me trouble. The results
> on them are not that definite to me: there are streaks of faint purple
> on my
> plates, not like the dark gram (-) purple results that I'm getting, nor
> are
> they like the ones with no growth. Anyone out there know whether I
> should
> assess these as gram+ or gram -? Thanks.
>> Microbio beginner,
> Harlequin
>
Harlequin,
My understanding of Eosin methylene blue plates is that the two
dyes INHIBIT the growth of Gram (+) bacteria, so ANY organisms growing
are Gram (-). The plates also contain lactose, so that determine those
Gram (-) buggies that ferment lactose (with isolated colonies: dark purple
colonies, usually in the very center of the colony, except E. coli,
which has a dark green metallic look to it) and those that do not. I'm
not sure about in between results like the one you describe. You can
always say +/- for lactose and use another biochemical test or two to
confirm which one it is; I'm sure some other more seasoned
microbiologist could answer this question. I assume that the plates were
made properly and/or purchased as is (Klebsiella pneumoniae or Enterobacter
cloacae could serve as positive controls; Shigella spp. as a negative
controls).
Are you identifying unkowns for a micro lab class? If so, it may be very
helpful to you to draw a flowchart of the most common bugs with the
simplest and easily readible (and fewest) biochemical tests that you can
perform. A flow chart will prevent you from doing needless tests and
give you an understanding of how the clin. microbiologists (used to) do
it. I found that doing this helped me to ID the bug much quicker and I
didn't have to make up as much media (and you just know how much we
love to make up media to ID the Gram (-) buggies!). I hope in this
course they introduce the quick test methods that many clinical labs
now use...
Have fun in your identification!
David
Remember to give thanks for those things you have which others do not!
Happy Thanksgiving!!
David J. McGee
MCGEED at hal.hahnemann.edu
Graduate Student
McPHU (Medical College of Pennsylvania and Hahnemann University)
Center City Campus
15th and Vine Streets
Dept. Microbiology and Immunology M.S. 410
New College Building 10302
Philadelphia, PA 19102
Phone: 215-762-8275
Fax: 215-762-1004
