SS Agar: Salmonella Shigella Agar

SS Agar (Salmonella Shigella Agar) is a differential selective media used for the isolation of Salmonella and some Shigella species from pathological specimens, and suspected foods. The peptic digest of animal tissue and beef extract provides essential growth nutrients. Lactose is a fermentable carbohydrate. Brilliant green, bile salts, and thiosulphate selectively inhibit gram-positive and coliform microorganisms. Sodium thiosulphate is reduced by certain species of enteric organisms to sulphite and H2S gas.

The production of H2S gas is detected as an insoluble black precipitate of ferrous sulphide, formed upon the reaction of H2S with ferric ions or ferric citrate. The fermentation of lactose results in the production of acid which is indicated by a change of color from yellow to red by the pH indicator, neutral red. Therefore, lactose-fermenting organisms grow as red-pigmented colonies. Lactose nonfermenting colonies grow as colorless colonies. Growth of the Salmonella species is uninhibited and appears as a colorless colony with a black center. Shigella grows as a colorless colony.

SS Agar composition

IngredientsGms / LitreIngredientsGms / Litre
Proteose peptone5.0Ferric citrate1.0
Lactose10.0Brilliant green0.33
Bile salts mixture8.5Neutral red0.25
Sodium citrate8.5Agar13.5
Sodium thiosulphate8.5Final pH7.0±0.2
SS Agar composition

SS Agar Principle

Differentiation of gram-negative bacilli on SS Agar is based on the fermentation of lactose and subsequent absorption of neutral red; gram-negative bacilli which ferment lactose produce pink to red colonies. Lactose-nonfermenters, such as Salmonella and Shigella, form transparent, colorless colonies. Bile salts, brilliant green, and citrates are selective agents that inhibit gram-positive bacteria and coliforms. Sodium thiosulfate, a sulfur source, and ferric ammonium citrate, an indicator, are added to enable organisms that produce H2S to form black-centered colonies, including some strains of Salmonella.

SS Agar Preparation

  • Suspend 63 grams of SS Agar in 1000 ml of 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 50°C. Mix and pour into sterile petri plates.

SS Agar Procedure

  • Inoculate and streak the specimen as soon as possible after it is received in the laboratory. Selective and nonselective media should be inoculated to increase the potential for isolation of enteric pathogens.
  • If material is being cultured directly from a swab, roll the swab over a small area of the agar surface. Streak the plate for isolation using a sterile inoculating loop.
  • Incubate the plates in ambient air at 33-37°C for 18-24 hours.
  • Examine the plate for growth and typical colony morphology. On SS Agar, colonies of Salmonella or Shigella are smooth and opaque or colorless.
  • Strains of Salmonella which produce H2S will form black-centered colonies. Lactose-fermenters are pink to rose-red in color and may have a precipitate.

SS Agar Result

Enteric organisms are differentiated by their ability to ferment lactose. Salmonella spp. and Shigella spp. are non-lactose fermenters and form colorless colonies on Salmonella Shigella Agar. H2S positive Salmonella spp. produce black-center colonies. Some Shigella spp. are inhibited on Salmonella Shige.


  • Organisms other than Salmonella and Shigella that grow on this medium may be differentiated by their ability to ferment lactose and form pink or red colonies.
  • A couple of non-pathogenic living beings might develop on Salmonella Shigella.
  • A few types of Shigella, for example, sonneiand S. dysenteriae serovar 1, may age lactose generally leisurely, and provinces change to lactose-maturing after development for at least 2 days.
  • SS Agar is not recommended for primary isolation of Shigella spp. because it is highly selective and may inhibit some strains.

SS Agar Expiration

Store dehydrated culture media at 2 – 30°C away from direct sunlight. Once opened and recapped, place the container in a low-humidity environment at the same storage temperature. Protect from moisture and light by keeping the container tightly closed.

SS Agar Storage

Refer to the expiration date stamped on the container. The dehydrated medium should be discarded if not free-flowing, or if appearance has changed from the original color.

SS Agar Quality Control

OrganismInoculumGrowthRecoveryColour of Colony
Escherichia coli50-100Fair20-30%Pink with bile precipitate
Enterobacter aerogenes50-100Fair20-30%Cream pink
Enterococcus faecalis50-100None-poor<=10%Colourless
Proteus mirabilis50-100Fair-good30-40%Colorless may have black
Salmonella choleraesuis50-100Good-luxuriant>=50%Colourless with
black centre
Salmonella Typhi50-100Good-luxuriant>=50%Colourless with
black centre
Salmonella Typhimurium50-100Good-luxuriant>=50%Colourless with
black centre
Salmonella Enteritidis50-100Good-luxuriant>=50%Colourless with
black centre
Shigella flexneri50-100GoodColor of ColonyColorless, may have black

What is the primary purpose of Salmonella Shigella Agar?

Salmonella Shigella Agar is designed for the isolation and differentiation of Salmonella and Shigella species in microbiological studies.

Can other agars be used interchangeably with Salmonella Shigella Agar?

While other agars exist, Salmonella Shigella Agar offers unique advantages, making it the preferred choice in many applications.

How often should quality control measures be implemented?

Regular quality control measures are crucial to maintaining the reliability of Salmonella Shigella Agar, and they should be conducted consistently.

What are common storage mistakes to avoid?

Avoid exposure to direct sunlight and extreme temperatures, as they can compromise the efficacy of Salmonella Shigella Agar.

Is Salmonella Shigella Agar suitable for environmental samples?

Yes, it can be used for environmental samples, but proper protocol and additional testing may be necessary.

Are there any safety precautions when working with Salmonella Shigella Agar?

Always follow laboratory safety guidelines, including the use of personal protective equipment and adherence to aseptic techniques.

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