Primary Purpose
Isolation of organisms like Salmonella and Shigella species
Type of medium
Selective (for Gram negative bacteria)
Differential (differentiates between a few organisms such as Salmonella, Shigella, Escherichia and Enterococcus etc)
Nutrients for growth
Peptones and Beef extract
What inhibits the Gram negative organisms?
Bile salts
Brilliant Green
Sodium citrate
How are lactose fermentors recognized?
The organisms which breakdown lactose produce acid. The acid production is identified by pink color because of the presence of the indicator Neutral red in the medium.
How are Salmonella and Shigella differentiated from other organisms?
As both are Lactose non-fermenting organisms they grow as colorless colonies with or without black centers.
How to differentiate between Salmonella and Shigella?
Salmonella produces H2S and therefore produces black centered colonies, while Shigella doesn't produce black centers.
Why are black centered colonies produced?
The medium contains Sodium thiosulphate and ferric citrate. Sodium thiosulphate is reduced to sulphite and H2S gas (due to enzyme thiosulphate reductase). Black precipitate of ferrous sulphide is formed when H2S reacts with ferric citrate.
pH of medium
7.0 +/- 0.2
Some facts about Salmonella Shigella Agar
-It is used to isolate Salmonella and Shigella from pathological specimens and suspected foodstuff.
-It is also known as SS agar or SS Medium
