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Salmonella Shigella (SS) Agar: Purpose, Principles and Uses

Welcome! You’re in the right place if you’re interested in delving into the fascinating world of microbiology or just curious about bacterial culture medium, especially the Salmonella Shigella (SS) Agar.

This guide will provide you with an extensive understanding of every aspect related to SS Agar – its composition, principles that drive it, purpose, and uses across different applications and steps involved in its preparation.

If we keep it simple – Salmonella Shigella (SS) Agar is a selective and differential culture medium usually used for isolating Salmonella and Shigella species from a sample specimen.

The specialized ingredients present in SS Agar help promote the growth of such bacteria while inhibiting other microorganisms – all thanks to the unique principle behind this fantastic scientific invention.

The Composition of Salmonella Shigella (SS) Agar

Salmonella Shigella (SS) Agar is a selective and differential medium primarily used for the isolation of pathogenic gram-negative bacteria, including Salmonella and Shigella species.

The Composition of Salmonella Shigella (SS) Agar

It consists of various ingredients, each playing a role in fostering the growth or differentiation of these bacteria. Let’s delve into the essential components and their purposes.

Essential Ingredients

The key elements that make up the SS Agar comprise:

  • Peptone and Beef Extract – These serve as sources of carbon, nitrogen, vitamins, and minerals necessary for bacterial growth.
  • Bile Salts – They inhibit non-pathogenic gram-positive organisms, permitting only gram-negative microbes to grow.
  • Lactose – A fermentable carbohydrate used by certain bacteria to release acid as end products, causing a change in agar color.
  • Neutral Red & Brilliant Green Dye – Neutral Red is a pH indicator dye that turns red under acidic conditions, while Brilliant Green inhibits gram-positive microbial growth.
  • Agar-Agar – It acts as the solidifying agent, providing structure to the medium.

Purpose of Each Component

Here’s how each component aids in separating strains of targeted bacterial species:

  • Peptone and beef Extract: Being rich nutrient sources, they foster all-around microbial development.
  • Bile Salts: Gram-positive bacteria are sensitive to them and hence can’t propagate or survive on this medium, which allows selective growth of Gram-negative microorganisms like Salmonella & Shigella species.
  • Lactose: Its fermentation leads to acid production, subjecting visual differentiation based on color change according to pH sensitivity.
  • Neutral Red & Brilliant Green Dye: Under acidic conditions triggered by lactose fermenters(like some harmless gut bacteria), neutral red turns the colony pinkish-red, whereas brilliant green hampers the growth of non-target, gram-positive microbes.
  • Agar: It provides a solid base for bacterial growth, making microbial colonies visible and enabling easy isolation.

The concerted action of these components allows SS agar to be selective (facilitating the growth of specified bacteria while inhibiting others) and differential (enabling clear visual variances amongst growing colonies).

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The Principle behind Salmonella Shigella (SS) Agar

The SS Agar or Salmonella-Shigella agar is an essential bacteriological medium used in microbiology labs, specifically designed to recover highly infectious bacteria like Salmonella and Shigella from a variety of clinical specimens.

It is a how-to guide for not only identifying these bacteria but also differentiating one from the other based on distinct biochemical processes.

Basics Principles

Salmonella-Shigella (SS) Agar operates on two essential principles:

  • Selection: SS Agar medium contains bile salts and brilliant green dye. These components inhibit the growth of Gram-positive bacteria and some Gram-negative bacteria, thus making it highly selective for certain types of bacterial species, including Salmonellae and Shigellae.
  • Differentiation: The presence of lactose and certain indicators within the SS agar plate allows for differentiation between colony types. Lactose fermenters will produce red or pink colonies, whereas lactose non-fermenters like Salmonellae and Shigellae will appear as colorless or transparent colonies.

These principles work hand in hand to isolate specific bacteria strands while preventing the growth of others that may create interference during testing.

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The Uses of Salmonella Shigella (SS) Agar

Salmonella Shigella Agar is highly utilized in microbiology for a variety of purposes. It’s primarily used in bacterial culture testing and scientific research due to its selective and differential properties. Let’s dive into these areas of use:

In Bacterial Culture Testing

Salmonella Shigella Agar is employed extensively in clinical microbiology laboratories all over the world. It serves a crucial role in detecting gram-negative bacteria, notably Salmonella and Shigella, from diverse specimens such as stool, urine, or wound swab samples.

  • Sample Detection: SS agar provides an efficient platform for growing bacteria once they’re extracted from the sample source. Its selective components inhibit unwanted microbes, allowing only targeted species to thrive.
  • Selective Growth: Due to constituents like bile salts and brilliant green dye that inhibit Gram-positive bacteria growth, SS agar is ideal for isolating difficult-to-culture pathogenic Gram-negative genera like Salmonella and Shigella.
  • Distinguishing Colonies: Different strains exhibit different colony colors on SS agar, which aids in quick identification. For instance, colonies from Salmonellas usually present as red with a black center, while those from most strains of shigellosis show up as red colonies without a black center.

In Scientific Research

Researchers also heavily utilize SS Aagar while conducting scientific projects targeting disease-causing pathogens where the identification or detection of bacteria forms an integral part of their studies.

  • Bacterial Studies: Many articles have been published using SS agar for researching various aspects, such as bacterial resistance patterns or epidemiological investigations on sources of transmission causing outbreaks.
  • Development Of New Tests: It has helped devise newer, better diagnostic tests such as salmonellosis using molecular techniques exploring processes at the cellular level through observation made on these plates cultures grown in this medium.

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Preparation Process Of Salmonella Shigella (SS) Agar

Preparing SS Agar requires a series of careful steps in order to create an ideal environment for the growth and isolation of salmonella and shigella bacteria.

Preparation Process Of Salmonella Shigella (SS) Agar

The procedure must be conducted under sterile conditions, with precise measurement of ingredients to ensure the optimal performance of the medium.

Steps Involved in Preparation

The preparation process involves combining all essential ingredients, sterilizing the mixture, and then pouring it into Petri dishes. Here are the detailed steps:

  • Measurement & Combination: Start by measuring out all necessary components – Lactose, bile salts, ferric citrate, sodium thiosulfate, neutral red phenol red dye, and agar. Note that each ingredient should be accurately weighed according to recommended quantities.
  • Sterilization: Once measured out correctly, mix these ingredients in distilled water until completely dissolved. The next step is sterilization using an autoclave at around 121°C for 15 minutes.
  • Cooling & Pouring: After sterilization is complete, cool the solution to about 45-50°C before adding necessary selective agents like brilliant green dye or sodium citrate as per requirement. Mix well before finally pouring into sterile Petri dishes.
  • Solidification & Storage: Once poured into Petri plates, allow it to solidify at room temperature (20-25°C). Store them upside down in the refrigerator at about 2–8°C until use, but make sure they are used within three weeks after preparation.

Result Interpretation (SS) Agar

Interpreting the results from Salmonella Shigella (SS) Agar involves observing the growth and color change of colonies on the medium. It is an essential step in differentiating between distinct bacterial species, particularly Salmonella and Shigella.

The table below summarizes this information:

OrganismColony MorphologyH2S Production
Salmonella SpeciesBlack/Dark-Centered Colonies; 2-4mm in sizePositive (H2S Produced)
Shigella SpeciesPale/White/Colorless ColoniesNegative (H2S Not Produced)
No GrowthNo Visible ColoniesMay Indicate absence of Salmonellae/Shigellae

Interpretations of these results are pivotal in the identification and discrimination between these two significant Enterobacteriaceae members. However, other confirmatory tests should also be considered to champion reliable, accurate diagnosis.

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Frequently Asked Questions

On which agar does Salmonella grow?

BS agar is the medium of choice for the isolation of Salmonella enterica serovar Typhi, and it is used for the isolation of atypical salmonellae, such as those that ferment lactose (7).

Is salmonella shigella agar solid or liquid?

Salmonella-Shigella Agar or SS Agar is a selective and differential medium for the isolation of pathogenic enteric bacilli, particularly Salmonella, Shigella, and Yersinia, from clinical samples.

What is the incubation period for salmonella shigella?

The incubation period is 12-96 hours, meaning that illness usually starts between 12 and 96 hours after shigella bacteria enter the body.

What media is used for salmonella shigella?

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 Salmonella Shigella (SS) Agar plays a vital role in bacterial studies and diagnostic processes, particularly distinguishing between Enterobacteriaceae members like salmonellae and shigellae, prevalent culprits behind foodborne illnesses.

Its unique composition helps differentiate these species based on their distinct metabolic capacities. The correct interpretation of outcomes is essential for accurate diagnosis, leading to timely treatment and avoiding potentially disastrous health ramifications.

However, it’s important to consider that such arrangements should be corroborated with other confirmatory tests for optimal accuracy.