Description
SS Agar – Salmonella Shigella Agar is recommended as a differential and selective medium for the isolation of Salmonella and Shigella species from pathological specimens and suspected foodstuffs and for microbial limit tests. It is a moderately selective medium in which gram-positive bacteria are inhibited by bile salts, brilliant green, and sodium citrate.
Composition of SS Agar:
- Peptone 5.000 Gms/Litre
- HM peptone B # 5.000 Gms/Litre
- Lactose 10.000 Gms/Litre
- Bile salts mixture 8.500 Gms/Litre
- Sodium citrate 10.000 Gms/Litre
- Sodium thiosulphate 8.500 Gms/Litre
- Ferric citrate 1.000 Gms/Litre
- Brilliant green 0.00033 Gms/Litre
- Neutral red 0.025 Gms/Litre
- Agar 15.000 Gms/Litre
- Final pH ( at 25°C) 7.0±0.2 Gms/Litre
- Formula adjusted, standardized to suit performance parameters
Directions of SS Agar:
Suspend 63.02 grams in 1000 ml purified /distilled water. Boil with frequent agitation to dissolve the medium completely.
DO NOT AUTOCLAVE OR OVERHEAT. Overheating may destroy the selectivity of the medium. Cool to about 50°C. Mix and
pour into sterile Petri plates.
Principle And Interpretation
SS Agar medium is recommended as a differential and selective medium for the isolation of Salmonella and Shigella species
from pathological specimens (1) and suspected foodstuffs (2,3,4,5) and for microbial limit test (6). SS Agar is a moderately
selective medium in which gram-positive bacteria are inhibited by bile salts, brilliant green and sodium citrate.
Peptone, HM peptone B provides nitrogen and carbon sources, long-chain amino acids, vitamins and essential growth
nutrients. Lactose is a fermentable carbohydrate. Brilliant green, bile salts and thiosulphate selectively inhibit gram-positive
and coliform organisms. Sodium thiosulphate is reduced by certain species of enteric organisms to sulphite and H2S gas and
this reductive enzyme process is attributed to thiosulphate reductase. Production of H2S gas is detected as an insoluble black
precipitate of ferrous sulphide, formed upon reaction of H2S with ferric ions or ferric citrate, indicated in the centre of the
colonies.
The high selectivity of Salmonella Shigella Agar allows the use of large inocula directly from faeces, rectal swabs or other
materials suspected of containing pathogenic enteric bacilli. On fermentation of lactose by a few lactose-fermenting normal
intestinal flora, acid is produced which is indicated by the change of colour from yellow to red by the pH indicator-neutral red.
Thus these organisms grow as red-pigmented colonies. Lactose non-fermenting organisms grow as translucent colourless
colonies with or without black centres. Growth of Salmonella species appears as colourless colonies with black centres
resulting from H2S production. Shigella species also grow as colourless colonies which do not produce H2S. Click to Know More
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