Liofilchem McFarland standard

McFarland standards are turbidity standards widely used in microbiology for preparing standardized suspensions of microorganisms. These suspensions are essential for performing antimicrobial susceptibility testing (AST), ensuring reproducibility and accuracy in laboratory procedures.

Description

McFarland standards are suspensions created by mixing barium chloride and sulphuric acid, resulting in a fine precipitate of barium sulphate that provides reproducible turbidity. This turbidity correlates with microbial density, making it a benchmark for preparing bacterial inocula.

Principle

1. Chemical Reaction:
   • Sulphuric Acid + Barium Chloride → Barium Sulphate Precipitate
   • This chemical reaction generates a stable, reproducible turbidity.
2. Calibration:
   • The turbidity of a bacterial suspension is visually compared to the McFarland standard or is used to calibrate an electrometric turbidimeter.
3. Application:
   • Standards are critical for ensuring consistent microbial densities during susceptibility testing and other microbiological assays.

Procedure

1. Preparation:
   • Before use, vortex the McFarland standard to achieve uniform turbidity.
2. Comparison:
   • Under proper lighting, visually compare the turbidity of the microbial suspension to the standard.
3. Electrometric Calibration (optional):
   • Use the McFarland standard to calibrate a turbidimeter for precise measurements.

Results Interpretation

The standards enable preparation of standardized inocula for use in:

• Kirby-Bauer disc diffusion testing.
• Broth or agar dilution methods.
• Standardized AST procedures as per CLSI guidelines.

McFarland Standards and Corresponding Microbial Densities

Note: These values are approximations for bacteria. For larger organisms like yeast, divide these values by ~30.

Applications

1. Antimicrobial Susceptibility Testing (AST):
   • Preparation of bacterial suspensions for standardized testing of antibiotics.
2. Microbial Enumeration:
   • Benchmark for counting colony-forming units (CFUs).
3. Yeast Studies:
   • Adjustments to account for larger microbial size in fungal assays.
4. Calibration:
   • Calibrate laboratory turbidimeters for precision in density measurement.

Product Range

Storage and Handling

• Storage Conditions:
  • Store at ambient temperature in a dry environment.
• Handling:
  • Ensure the standards are agitated before use to maintain uniform turbidity.

Precautions

1. Handle with care, as the standards are fragile.
2. Use only for in vitro diagnostic purposes.
3. Follow standard laboratory protocols for microbial handling.

Advantages

1. Accuracy:
   • Provides precise microbial density standards.
2. Versatility:
   • Suitable for bacterial and yeast suspensions.
3. Convenience:
   • Ready-to-use standards simplify laboratory workflows.
4. Compliance:
   • Aligns with CLSI and EUCAST guidelines for susceptibility testing.

Thermo Scientific™ Remel™ McFarland Standards vs. Liofilchem® McFarland Standards

Key Differences

1. Material: Thermo Scientific standards are in polystyrene tubes for extended durability, while Liofilchem uses glass for precision.
2. Packaging: Thermo Scientific provides a visual comparison card in a portable plastic case, whereas Liofilchem standards are individually packaged.
3. Range: Both brands cover 0.5–4.0 standards; however, Thermo Scientific offers an additional 5.0 standard.

This comparison highlights the strengths and specific use cases of both product lines.

The Liofilchem McFarland Standards are indispensable tools for microbiological laboratories, ensuring reproducibility and accuracy in bacterial suspension preparation. Their wide range and compatibility with both visual and electronic methods make them versatile and reliable for AST and other diagnostic applications.

References

1. McFarland, J. A. M. A. (1907). Vol. 49: 1176.
2. Bailey & Scott's Diagnostic Microbiology, 13th Edition (2014).
3. CLSI M7-A9 (2012) - Dilution Methods for Aerobic Bacteria.
4. CLSI M11-A7 (2007) - Dilution Methods for Anaerobic Bacteria.