In a proficiency exercise, one technician prepares the inoculum at approximately 2 McFarland instead of the required 0.5 McFarland before performing Kirby-Bauer disc diffusion on a fully susceptible E. coli isolate. What systematic error will result?

• A Only the innermost colonies inside zones will disappear
• B Zones will be uniformly larger than QC ranges, potentially reporting a resistant isolate as susceptible
• C Zone diameters remain valid because disc content dominates diffusion
• D Zones will be uniformly smaller than QC ranges, potentially reporting a susceptible isolate as resistant ✓

Explanation

Zone size depends on the balance between antibiotic diffusion and organism growth rate. D heavier-than-standard inoculum yields denser lawn growth, producing smaller inhibition zones across all discs and risking false categorisation of susceptible isolates as resistant. D too-light inoculum does the reverse, giving falsely large zones. Disc content sets the gradient but cannot correct for an overloaded lawn, so QC with ATCC 25922 is mandatory each run.

Reference: Bailey and Scott's Diagnostic Microbiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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