McFarland Standards- Principle, Preparation, Uses, Limitations


Microbiology laboratories often need to measure the concentration or density of bacterial cells in a liquid suspension. For example, when performing antimicrobial susceptibility testing or quality control of culture media, it is important to use a standardized inoculum of bacteria that is comparable across different samples and tests. However, counting the number of bacteria in a suspension can be time-consuming, labor-intensive and require specialized equipment. Therefore, microbiologists use a simple and convenient method to estimate the bacterial density by comparing the turbidity or cloudiness of the suspension to a reference solution.

This reference solution is called a McFarland Standard. It is a chemical solution that produces a fine precipitate of barium sulfate when mixed in certain proportions. The turbidity of a McFarland Standard is visually similar to that of a bacterial suspension with a known concentration. By using different volumes of the chemical reagents, McFarland Standards of different turbidity levels can be prepared, which correspond to different bacterial densities. The most commonly used McFarland Standard for microbiology applications is the 0.5 McFarland Standard, which has a turbidity equivalent to a bacterial suspension with about 1.5 x 10^8 colony forming units (CFU) per milliliter.

To use a McFarland Standard, the microbiologist prepares a bacterial suspension from a pure culture and adjusts its turbidity by adding broth or saline until it matches the appearance of the McFarland Standard. Then, the microbiologist can use the standardized bacterial suspension for further testing or analysis. This method allows for a quick and easy estimation of bacterial density without requiring sophisticated instruments or calculations.

McFarland Standards are widely used in microbiology laboratories for various purposes, such as:

  • Antimicrobial susceptibility testing: To determine the sensitivity or resistance of bacteria to different antibiotics, the microbiologist inoculates agar plates with standardized bacterial suspensions and observes the zones of inhibition around antibiotic disks.
  • Quality control of culture media: To ensure the performance and reliability of culture media, the microbiologist inoculates them with standardized bacterial suspensions and checks for growth, color change and other indicators.
  • Identification of bacteria: To identify unknown bacteria based on their biochemical or physiological characteristics, the microbiologist inoculates different test media or reagents with standardized bacterial suspensions and observes the reactions.

McFarland Standards are an essential tool for microbiology laboratories that need to work with consistent and comparable bacterial suspensions. They are simple to prepare and use, and they provide a reasonable approximation of bacterial density for various applications. However, they also have some limitations that need to be considered, such as:

  • The accuracy and reproducibility of McFarland Standards depend on several factors, such as the quality and age of the chemical reagents, the storage conditions, the lighting and contrast for visual comparison, and the type and size of the tubes used for the suspensions.
  • The turbidity of McFarland Standards may not match exactly with the bacterial density due to variations in cell size, shape and aggregation among different bacterial species and strains.
  • The turbidity of McFarland Standards may not be suitable for colored media or reagents that can interfere with the visual comparison.
  • The turbidity of McFarland Standards may not reflect the viability or activity of the bacteria in the suspension, which may affect the results of some tests.

Therefore, McFarland Standards should be used with caution and care, and they should be validated and verified regularly with other methods of measuring bacterial density, such as spectrophotometry or colony counting.