DES O'CONNOR wrote in message <01bdacbd$e1290440$998066c3 at default>...
>There are two sensible reasons for the inclusion of Lactose :-
>>a) There are strains of Lactose positive Salmonella
If this were a reason, then the method would have been designed for the 0.8%
lactose-positive strains, which is not.
>b) Lactose can off set the toxic effects of bile salts.
The pre-enrichment in lactose broth is conducted before enrichment in media
with bile salts, therefore, this is no good reason either.
Old FDA (don't ask me for the reference now, I will look for it) indicates
that if coliforms are also present in the sample, they will ferment so
lowering the pH, which will rapidly create a self-contained environment. In
the mean time, Salmonella (especially stressed cells) will have grown -
just a couple of generations, necessary to repair any sublethal damage in
stressed cells-. Then, you transfer to enrichment broths that will really
select for Salmonella. Another facts about lactose broth are:
1. Lactose broth is a good propagation medium so, lactose will not play any
role in growth of Salmonella unless coliforms are present (see above
comment).
2. Lactose broth is cheap and usually most labs use it for other purposes
(convenient)
3. D'Aoust and Maishment, J. Food Prot. 42:153. 1979 reported that lactose
broth does not differ from other 9 general-use broths in pre enriching for
Salmonella in a variety of foods, therefore lactose in lactose broth may no
be playing any role in Salmonella inhibition or pre enrichment.
ALEX
