>> 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?
Please note that Brilliant Green agar does _not_ support growth of
Salmonella typhi (whereas Hektoen does). A few % of salmonellae do no
produce H2S; they readily grow on Hektoen but fail to produce black
centers. The most important difference between BG and Hektoen is that BG
usually is more selective, i.e. better suppresses competing flora. Thus
BG is usually recommended for specimens with a heavy flora. It may,
however, sometimes be too selective and suppress growth of salmonellae
too. Therefore it is advisable to use both a strongly selective as well
as a less selective (like Hektoen) medium in parallel.