The Differential And Selective Media For Salmonella And Shigella

 

Salmonella Shigella Agar (SS Agar) medium is recommended for the isolation of Salmonella and Shigella species from pathological specimens, suspected foodstuffs, and microbial limit tests. The selective inhibitory components brilliant green, bile salts, thiosulphate, and citrate inhibit gram-positive and coliform bacteria. Enteric organisms, on the other hand, are differentiated by the addition of lactose to the medium. Lactose-fermenting organisms produce acid, which results in the formation of red colonies in the presence of a neutral red indicator. Lactose non-fermenter colonies are colourless. The majority of intestinal pathogens, including Salmonella and Shigella, are found in the latter group. The sodium thiosulfate and ferric citrate allow the detection of hydrogen sulphide production, as evidenced by colonies with black centres. This highly selective formulation is not recommended for primary Shigella isolation.

Principle

The high selectivity of Salmonella Shigella Agar allows for the use of large inocula obtained directly from faeces, rectal swabs, or other materials suspected of containing pathogenic enteric bacilli. Lactose fermentation by a few lactose-fermenting normal intestinal flora produces acid, which is indicated by the pH indicator-neutral red changing colour from yellow to red. As a result, these organipigmented-pigmented colonies. Lactose-nonfermenting organisms grow as colourless colonies with or without black centres. Certain enteric organisms reduce sodium thiosulphate to sulphite and H2S gas, and this reductive enzyme process is attributed to thiosulphate reductase. The presence of H2S gas is detected in the colonies as an insoluble black precipitate of ferrous sulphide formed by the reaction of H2S with ferric ions or ferric citrate. Salmonella species grow as colourless colonies with black centres caused by H2S production. Shigella species can also grow as colourless colonies that do not emit H2S.

Composition

Peptone and beef extract provide nitrogen and carbon sources, long-chain amino acids, vitamins, and essential growth nutrients. The fermentable carbohydrate is lactose. Brilliant green, bile salts, and thiosulphate selectively inhibit gram-positive and coliform organisms.


              Ingredients

    Gms/Ltr

Peptone

5.000

Beef extract

5.000

Lactose

10.000

Bile salts mixture

8.500

Sodium citrate

10.000

Sodium thiosulphate

8.500

Ferric citrate

1.000

Brilliant green

0.00033

Neutral red

0.025

Agar

15.000

 

How To Prepare?

  • In 1000 mL of distilled water, dissolve 63.02 grams of TM Media dehydrated Salmonella Shigella agar.
  • To completely dissolve the medium, bring it to a boil with frequent agitation; 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.
  • In Petri dishes, a clear to slightly opalescent reddish-orange gel forms.

Colony Characteristics

Microorganism

ATCC

Inoculum (CFU/ml)

Colony Colour

Escherichia coli

25922

50-100

Pink with bile precipitate

Salmonella typhi

6539

50-100

Colourless with black centre

Proteus mirabilis

25933

50-100

Colourless, may have black centre

Shigella flexneri

12022

50-100

Colourless

At TM Media, we offer a wide range of culture media for bacterial or fungal isolation. Along with Culture Media, we also provide Laboratory Consumables, Biological Media Bases, Media Supplements, Antibiotic Sensitivity Discs, and many more.

For more, visit https://www.tmmedia.in/.

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