21/01/2026
McFarland Standards in AST
โก๏ธIn microbiology, McFarland Standards are turbidity standards used to visually match the cloudiness (turbidity) of a bacterial suspension to a known cell concentration, most famously for standardizing inoculum for antibiotic susceptibility testing (like the Kirby-Bauer method) and other assays, ensuring consistent bacterial counts (e.g., 0.5 McFarland โ 1.5 x 10โธ CFU/mL) for accurate results, often prepared with barium sulfate precipitate or modern latex particles.
โก๏ธA Turbidity Scale: A set of reference solutions with varying cloudiness, representing known concentrations of bacteria.
โก๏ธComposition: Originally barium sulfate (from barium chloride and sulfuric acid) or now often latex particles in a buffer, suspended to create light scattering.
โก๏ธNumbering: Standards are numbered (e.g., 0.5, 1, 2, 4), with higher numbers indicating greater turbidity and bacterial count.
๐งซHow They're Used
โก๏ธPrepare Bacteria: Grow bacteria and suspend colonies in sterile saline or broth.
โก๏ธCompare Turbidity: Hold the bacterial suspension tube next to the McFarland standard tube (e.g., 0.5 McFarland).
โก๏ธAdjust Concentration: If the bacterial suspension is clearer (less turbid), add more bacteria or concentrate it.
If it's cloudier (more turbid), dilute it with sterile broth/saline.
โก๏ธStandardized Inoculum: Once matched, the bacterial suspension has a consistent, known concentration (e.g., ~1.5 x 10โธ CFU/mL for 0.5 McFarland) for tests like antibiotic susceptibility.
Post Credit: Umar Khan, QAU.
