Salmonella count

Fri Aug 14 07:46:27 EST 1998

Dear Roberta,

                     I hope the following information helps. Salmonella can
be quite difficult to enumerate due to poor recovery on solid media without
any pre-enrichment or resuscitation stage. Some years ago a group at
Barcelona or perhapes, Madrid University published papers reviewing
comparative methods of recovering Salmonella on solid media; you could
trace that down.

My suggestions are :-

1) Surface plating (37' c 2 days) on Tryptone soya or Columbia agar. Follow
this with replica platting of the incuated plates onto X.L.D. and or
Bismuth Sulphite agar. This approach permits non selective resusciatation
followed by replication of colonies on differential media. You will of
course still have to conducted confirmatroy serology and biochemistry. 
I've tried this and I consider it is a good approach.

2) A less accurate system could be developed along the lines of the MPN
technique commonly used for enumeartion of of Coliforms in water. It might
be possible to develop a system based on a suitabley selective self
indicating broth. I believe there is such a system called L.I.N.C.R. which
was developed some years ago and may be still avialable through Oxoid or

The latter ( I think) is basically a Lysiene decarboxylase broth containing
a sensitive H2S indicator. At 42'c premuptively positive dilutions of food
turn black. Such tubes may be subbed to selective media for serological and
biochemical confirmation. MPN table may then be used to produce a most
probabale level of contamination.


Des O'c

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