Thermo Scientific™ Oxoid™ Ss Agar (Dehydrated), 500G

Thermo Scientific™ Oxoid™ SS Agar is a selective and differential culture medium designed for the isolation and identification of Salmonella and Shigella species from clinical specimens, food samples, or environmental sources. It is formulated to differentiate these pathogenic bacteria based on lactose fermentation and hydrogen sulfide (H2S) production, while inhibiting the growth of most Gram-positive and coliform organisms.

Key Features

1. Differential Characteristics:
   • Lactose Fermentation: Lactose-fermenting organisms produce red colonies due to acid production interacting with the neutral red indicator.
   • Non-Lactose-Fermenting Organisms: Salmonella and Shigella form transparent or straw-colored colonies.
2. H2S Detection:
   • Thiosulfate in the medium reacts with iron, producing black-centered colonies for organisms capable of hydrogen sulfide production, such as some Salmonella species.
3. Selective Inhibition:
   • Contains brilliant green, bile salts, thiosulfate, and citrate, which inhibit Gram-positive and coliform organisms, enhancing selectivity for Salmonella and Shigella.
4. Ease of Differentiation:
   • Provides clear visual differentiation between lactose fermenters (red colonies) and non-fermenters like Salmonella and Shigella (straw-colored colonies).

Applications

• Clinical Microbiology:
  • Isolation of Salmonella and Shigella from fecal specimens and rectal swabs.
• Food Safety Testing:
  • Detection of pathogenic Salmonella and Shigella in contaminated food products like meat, poultry, and produce.
• Environmental Monitoring:
  • Analysis of water samples to identify contamination by Salmonella or Shigella.

Composition

• Selective Agents: Brilliant green, bile salts, thiosulfate, and citrate inhibit non-target organisms.
• Differentiation Indicators:
  • Lactose as the fermentable carbohydrate.
  • Neutral red as the pH indicator for lactose fermentation.
  • Thiosulfate and iron for H2S detection.

Test Procedure

1. Preparation: Dissolve the dehydrated medium in distilled water, autoclave as per the product instructions, and pour into sterile petri dishes.
2. Inoculation: Streak the sample directly onto the prepared SS Agar plate.
3. Incubation: Incubate plates aerobically at 35–37°C for 18–24 hours.
4. Observation:
   • Red colonies: Lactose fermenters.
   • Straw-colored colonies: Non-lactose fermenters (Salmonella, Shigella).
   • Black-centered colonies: H2S producers (e.g., some Salmonella species).

Result Interpretation

• Lactose Fermenters (e.g., Escherichia coli): Red colonies due to acid production.
• Non-Lactose Fermenters:
  • Salmonella: Straw-colored colonies, often with black centers if H2S is produced.
  • Shigella: Transparent, straw-colored colonies without black centers.
• Inhibited Growth: Most Gram-positive and coliform organisms are suppressed.

Advantages

1. High Selectivity: Effective inhibition of Gram-positive and coliform bacteria.
2. Differentiation Capabilities: Easily distinguishes lactose fermenters from non-lactose fermenters and detects H2S production.
3. Versatility: Applicable for clinical, food, and environmental testing.

Limitations

• Some strains of Shigella and Salmonella may exhibit atypical colony morphology or delayed growth under specific conditions.
• Overgrowth by partially inhibited coliforms may occur if the sample is heavily contaminated.

Storage and Stability

• Store the dehydrated medium at 2–8°C in a cool, dry place.
• Prepared plates should be stored under refrigeration and used within the recommended timeframe.

Thermo Scientific™ Oxoid™ SS Agar (Dehydrated) is a powerful tool for the reliable isolation and differentiation of Salmonella and Shigella species, supporting clinical diagnostics, food safety, and environmental monitoring workflows.