Thermo Scientific™ Oxoid™ Sorbitol MacConkey Agar (Dehydrated)

The Thermo Scientific™ Oxoid™ Sorbitol MacConkey Agar is a specialized modification of MacConkey Agar No. 3, designed specifically for the selective detection and isolation of Escherichia coli O157. By replacing lactose with sorbitol as the carbohydrate substrate, this medium leverages the inability of E. coli O157 to ferment sorbitol, allowing clear differentiation from other E. coli strains.

Key Features

1. Selective for E. coli O157

• Incorporates sorbitol instead of lactose:
  • E. coli O157: Produces colorless colonies due to the inability to ferment sorbitol.
  • Other E. coli strains: Produce pink colonies due to sorbitol fermentation.

2. Efficient and Reliable

• Recommended as a simple, inexpensive, and rapid screening tool for E. coli O157.
• Sensitivity reported as 100% and specificity as 85% (March and Ratnam, 1986).

3. High Selectivity

• Bile Salts No. 3 and crystal violet inhibit Gram-positive organisms, enhancing selectivity for Gram-negative bacteria.
• Supports robust growth and differentiation of enteric bacteria.

4. Application Versatility

• Useful in clinical, food, and environmental microbiology for identifying E. coli O157 in:
  • Stool samples (clinical diagnostics).
  • Food samples (e.g., beef, dairy).
  • Water and environmental samples.

5. Disease Significance

• E. coli O157 is a key pathogen causing:
  • Hemorrhagic colitis (bloody diarrhea, severe abdominal pain).
  • Hemolytic uremic syndrome (HUS), a severe complication affecting kidneys.

Composition (Typical Formula per Liter)

Applications

1. Clinical Diagnostics

• Detect E. coli O157 in stool samples from patients with symptoms of hemorrhagic colitis or HUS.

2. Food Microbiology

• Screen food products such as:
  • Raw or undercooked beef.
  • Dairy products.
  • Other potentially contaminated food items.

3. Environmental Testing

• Monitor water supplies and environmental samples for contamination with E. coli O157.

Advantages

1. High Sensitivity

• A reliable tool with 100% sensitivity for detecting E. coli O157, ensuring no false negatives.

2. Clear Differentiation

• Easy to distinguish between:
  • Pathogenic E. coli O157 (colorless colonies).
  • Non-pathogenic E. coli strains (pink colonies).

3. Cost-Effective

• A practical and economical alternative for routine screening compared to molecular methods.

4. Broad Applicability

• Suitable for clinical, food, and environmental laboratories.

5. APHA Recommended

• Recognized for its efficiency and reliability in isolating E. coli O157.

How It Works

1. Preparation:
   • Rehydrate the dehydrated powder as per manufacturer instructions.
   • Autoclave and pour into sterile Petri dishes.
2. Inoculation:
   • Streak the sample (e.g., stool, food, or water) onto the agar surface.
3. Incubation:
   • Incubate plates at 35–37°C for 18–24 hours under aerobic conditions.
4. Result Interpretation:
   • Observe colony morphology:
     • Colorless colonies: E. coli O157 (non-sorbitol fermenters).
     • Pink colonies: Other E. coli strains (sorbitol fermenters).

Complementary Testing

• Confirmatory Testing:
  • Further biochemical or molecular testing (e.g., PCR) may be required to confirm E. coli O157.
• Complementary Media:
  • MacConkey Agar: For general coliform differentiation.
  • CHROMagar™ O157: For enhanced visualization of E. coli O157 colonies.

Why Choose Sorbitol MacConkey Agar?

• Efficient Detection: Highly sensitive and specific for E. coli O157.
• Cost-Effective Screening: Ideal for routine diagnostics without expensive equipment.
• Ease of Use: Provides clear, easy-to-read results for rapid identification.

The Thermo Scientific™ Oxoid™ Sorbitol MacConkey Agar is an essential tool for laboratories focused on public health, food safety, and environmental microbiology.

Differences Among MacConkey Agar Variants

MacConkey Agar has several modified formulations, each tailored for specific microbiological applications. These variants differ in their composition, selectivity, and intended use. Below is a detailed comparison of the major MacConkey Agar types:

1. Standard MacConkey Agar

• Purpose: General-purpose medium for the detection and differentiation of lactose-fermenting and non-lactose-fermenting Gram-negative bacteria.
• Selectivity:
  • Selective for Gram-negative bacteria.
  • Inhibits most Gram-positive organisms using bile salts and crystal violet.
• Key Features:
  • Differentiates lactose fermenters (pink colonies) from non-lactose fermenters (colorless colonies).
  • Commonly used for clinical and food microbiology.
• Applications:
  • Isolation of Escherichia coli and other coliforms from clinical and environmental samples.

2. MacConkey Agar No. 2

• Purpose: Specifically designed for the detection of enterococci in the presence of coliforms and non-lactose fermenters.
• Selectivity:
  • Enhanced selectivity for enterococci using Oxoid Bile Salts No. 2 and crystal violet.
  • Inhibits non-fecal streptococci and bile-tolerant Gram-positive cocci.
• Key Features:
  • Enterococci appear as small, intensely red colonies with pale periphery.
  • Non-lactose fermenters appear colorless.
• Applications:
  • Monitoring water, sewage, and food products for fecal contamination.

3. MacConkey Agar No. 3

• Purpose: Optimized for detecting and enumerating coliform organisms, while isolating Salmonella and Shigella species in food, water, and clinical samples.
• Selectivity:
  • Incorporates Oxoid Bile Salts No. 3 and crystal violet for improved selectivity.
  • Completely inhibits Gram-positive cocci.
• Key Features:
  • E. coli: Red colonies with bile precipitation.
  • Salmonella and Shigella: Straw-colored colonies.
  • Recommended by the APHA for food and water testing.
• Applications:
  • Pathogen detection in food (e.g., cheese) and environmental samples.

4. Sorbitol MacConkey Agar (SMAC)

• Purpose: Differentiation of E. coli O157, which does not ferment sorbitol.
• Selectivity:
  • Replaces lactose with sorbitol as the carbohydrate source.
  • Differentiates sorbitol-negative E. coli O157(colorless colonies) from other E. coli strains (pink colonies).
• Key Features:
  • Essential for detecting pathogenic E. coli in food and clinical samples.
• Applications:
  • Food safety testing for shiga toxin-producing E. coli (STEC).
  • Diagnostic testing for E. coli O157in fecal samples.

5. MacConkey Agar with MUG

• Purpose: Detects E. coli using the fluorescence of *MUG (4-Methylumbelliferyl-β-D-glucuronide) under UV light.
• Selectivity:
  • Contains MUG as an indicator for β-glucuronidase activity.
• Key Features:
  • E. coli fluoresces under UV light due to enzymatic activity.
  • Combines lactose differentiation with enzymatic detection.
• Applications:
  • Rapid identification of E. coli in clinical and environmental microbiology.

6. Crystal Violet MacConkey Agar

• Purpose: Enhances selectivity for coliforms by including a higher concentration of crystal violet.
• Selectivity:
  • Strong inhibition of Gram-positive organisms.
• Key Features:
  • Similar to standard MacConkey Agar but with increased inhibition of Gram-positive bacteria.
• Applications:
  • Used in highly contaminated samples to minimize Gram-positive interference.

Key Differences Summary

Conclusion

Each MacConkey variant is optimized for specific applications:

• Standard MacConkey Agar: General-purpose use.
• MacConkey Agar No. 2: Enhanced for enterococci detection.
• MacConkey Agar No. 3: Improved differentiation for Salmonella and Shigella.
• Sorbitol MacConkey Agar (SMAC): Essential for detecting E. coli O157.
• MacConkey with MUG: Combines enzymatic activity detection with traditional differentiation.

Selecting the appropriate variant depends on the target organism and sample type, ensuring accurate and efficient diagnostics.