Antibiotic bioassay — cylinder plate method for penicillin: Early official assays measured penicillin potency by the quantity needed to prevent Staphylococcus aureus growth, producing what standard inhibition-zone diameter on a cylinder-plate assay?

Introduction / Context:
Before modern MIC determinations, antibiotic potency was quantified by standardized agar diffusion bioassays. The cylinder-plate method compared unknown samples to reference standards based on reproducible inhibition zones.

Given Data / Assumptions:

• Assay organism: Staphylococcus aureus.
• Method: cylinder-plate assay (agar diffusion with antibiotic-filled cylinders).
• Question: the target zone diameter historically specified for penicillin potency.

Concept / Approach:
In the classic method, potency correlates with the diameter of the growth-free zone surrounding the antibiotic source. The 20 mm diameter is a commonly cited standard target in foundational pharmacopeial procedures for penicillin activity comparison under controlled conditions (medium, inoculum, incubation).

Step-by-Step Solution:
Recall the standardized zone diameter used historically in the cylinder-plate penicillin assay.Recognize that a 200 mm zone is unrealistic for plate size, and 24 mm was not the reference value in the classic protocol.Select the historically referenced 20 mm diameter.

Verification / Alternative check:
Old pharmacopeial and industrial QC manuals document calibration curves linking dose to zone diameters near this target.

Why Other Options Are Wrong:
Values too large or mismatched to the classical specification; “None” is incorrect as a standard target did exist.

Common Pitfalls:
Confusing cylinder-plate assays with disk diffusion; while similar in principle, their calibrations differ.

Final Answer:
20 mm diameter zone.