In susceptibility testing, the suspension is standardized to match the turbidity of which McFarland standard?

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

Answer: (B)

Standardizing the inoculum to match the 0.5 McFarland turbidity is used in susceptibility testing. This level of turbidity corresponds to roughly 1.5 × 10^8 CFU/mL, giving a reproducible bacterial lawn for reliable diffusion of the antibiotic. By adjusting the suspension to visually resemble the 0.5 McFarland standard, you ensure consistent inoculum density across tests, which helps zone sizes or MIC readings match interpretive criteria. Using a higher turbidity would produce more cells and smaller zones (potential false resistance), while a lower turbidity would yield fewer cells and larger zones (potential false susceptibility).