A suspension of the test organism for broth dilution and disk diffusion testing is adjusted to match the turbidity of which McFarland standard?

• A. #0.5 McFarland
• B. #1.0 McFarland
• C. #2.0 McFarland
• D. #3.0 McFarland

Answer: (A)

Standardizing the number of bacteria in the inoculum is essential for reproducible antimicrobial susceptibility testing. Turbidity standards provide a visual reference to achieve a consistent cell density across tests. For both broth dilution and disk diffusion, the suspension is adjusted to match the 0.5 McFarland standard. This level corresponds to roughly 1.5 x 10^8 CFU/mL, giving a dense but manageable lawn of bacteria that yields reliable MIC values and zone diameters. If the suspension were more turbid, you’d get smaller zones (or higher MICs) due to an overly heavy inoculum; if it were less turbid, zones would be larger and results could appear falsely susceptible. Therefore, matching to 0.5 McFarland is the appropriate and standard choice.