Chapter 5.14 : Preparation of Routine Media and Reagents Used in Antimicrobial Susceptibility Testing

Abstract:

McFarland turbidity standards are used to standardize the approximate number of bacteria in a liquid suspension by visually comparing the turbidity of a test suspension with the turbidity of a McFarland standard. McFarland standards are prepared by adding barium chloride to sulfuric acid to obtain a barium sulfate precipitate. By adjusting the volumes of these two reagents, standards of varying degrees of turbidity can be prepared to represent several different concentrations of bacteria. The standard most commonly used in the clinical microbiology laboratory for routine antimicrobial susceptibility tests ( 1 , 2 ) is 0.5, which represents 1.5 × 108 (generally, range is 1.0 × 108 to 2.0 × 108) bacteria/ml. McFarland standards are commercially available from several sources. As an alternative to the traditional barium sulfate standards, McFarland standards prepared from latex particles have recently become commercially available.