Johnny Barone writes,
<Recently, I discovered that both the United States Pharmacopia Compendia
and the European Pharmacopia Compendia are proposing the use of Hektoen
Enteric Agar instead of Brilliant Green Agar for Salmonella detection.
I cannot determine the reason for this change because both agars seem to
select for Salmonella sp. just as well.
Does anyone have an explaination for this change?>
Wow, good question! I regularly QC test both of these media when they
come in to the lab I'm working at presently. I just popped open my Difco
Manual, and according to this, the immediate impression I got was that
Brilliant Green is actually slightly better - S. typhi (ATCC #19430), S.
enteritidis (ATCC #13076) *and* S. typhimurium (ATCC # 14028) will all be
recovered on BG, whereas only the latter two organisms are said to be
recovered on HE. The lab I work in now only maintains the last of these
three organisms in stock culture (I just made up a new freeze culture of
this a couple of weeks ago, in fact!). I use it for testing both of these
media, and not surprisingly, it grows very well on each.
However, on closer examination of the information in my Difco manual, I
would ask if it's possible that the sodium thiosulfate and ferric
ammonium citrate in HE allow for differentiation between species of
Salmonella which produce H2S and those which don't. The S. typhimurium I
use produces blue-green colonies with black centers (it's an H2S
producer) on HE, and on BG I get the pinkish white colonies and red agar.
All Salmonella sp. listed for BG produce pinkish white colonies and
redden the agar (due to the phenol red in this medium). One more thing,
Shigella will grow on HE but is not said to grow on BG....
But these are the only differences I notice between the two media. Do you
(or does anyone else in here) know if H2S producing Salmonella sp. are
"worse" than the non-producers?
Do the United States Pharmacopia Compendia and the European Pharmacopia
Compendia state any reasons for this proposal?
--Yersinia.
