Salmonella-Shigella (SS) Agar: Principle, Composition ,Preparation,Inoculation and colony morphology

SS AGAR

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What is Salmonella-Shigella Agar

SS Agar is recommended for use as a selective and differential medium for the isolation of Salmonella and some Shigella species from clinical and non-clinical specimens.
It was originally developed as a selective medium for the isolation of Salmonella and Shigella species. It was also developed to aid in the differentiation of lactose and non-lactose-fermenters from clinical specimens, suspected foods, and other such samples.
Despite the name, Salmonella-Shigella or SS agar,It is not suitable for isolating shigellae as it is inhibitory to most of it's strains.

What is the Principle of Salmonelle-Shigella Agar

The basis for differentiation on Salmonella Shigella Agar depends on the ability to ferment lactose and the absorption of neutral red as the bile salts precipitate in the acidic condition. Neutral red turns red in the presence of an acidic pH produced by bacteria, thus showing fermentation has occurred.
The inclusion of bile salts, sodium citrate, and brilliant green serve to inhibit gram-positive and coliform organisms. Salmonella, Shigella, and other non-lactose-fermenting organisms appear as transparent or translucent colorless colonies on SS Agar.
Sodium thiosulfate is added to the medium as a hydrogen sulfide source, and ferric citrate is added as an indicator for hydrogen sulfide production.
While the Agar is the solidifying gel.

What are the composition of Salmonella-Shigella Agar

Composition of ss agar

Ingredients per liter of deionized water:

Lactose 10.0gm
Bile Salts No. 3 8.5gm
Sodium Citrate 8.5gm
Sodium Thiosulfate 8.5gm
Beef Extract 5.0gm
Proteose Peptone 5.0gm
Ferric Citrate 1.0gm
Brilliant Green 0.33mg
Neutral Red 0.025gm
Agar 13.5gm

Final pH obtain is 7.0 +/- 0.2 at 25ºC.

How to Prepare Salmonella Shigella Agar

Suspend 60 g of the medium in one liter of deionized if available or distilled water.
Mix well.
Heat with frequent agitation and boil for one minute.
Sterilization in autoclave is not necessary.
Pour into plates
Let the agar solidify and store in the refrigerator (avoid freezing). Prepared culture media can be kept for at least a week in refrigeration.

Note: Various commercial suppliers now supplies ready to use culture plates.

SS Agar should appear clear, and red-orange in color.

Sample Collection: Infectious material or sample should be submitted directly to the laboratory without delay and protected from excessive heat and cold. If there is to be a delay in processing, the sample should be inoculated onto an appropriate transport media and refrigerated until inoculation.

How to Inoculate and Culture on Salmonella Shigella Agar

After preparation,

Allow the plates to warm to room temperature and the agar surface to dry before inoculating.
Heavily inoculate and streak the specimen as soon as possible after collection.
If the sample to be cultured is on a swab, roll the swab over a small area of the agar surface.
Streak for isolation with a sterile loop.
Incubate plates aerobically at 35-37 degrees C. for 18-24 hours.
Examine colonial morphology.

How to interprete colonies after culture on SS AGAR

If there is lactose fermentation , the medium will turn red due to the acidic pH. E.g. Escherichia coli, Klebsiella pneumoniae
Salmonella, Shigella,and other non-lactose fermenters appear as transparent or translucent colorless colonies on SS Agar.
Colonies of Salmonella spp. may appear with or without black centers.

However , It is recommended that biochemical, immunological, molecular, or mass spectrometry testing be performed on colonies from pure culture for complete identification and conclusion.