Which Oxford researcher developed a penicillin assay using agar nutrient plates?

Norman Heatley developed the penicillin assay that used bacteria-seeded agar nutrient plates.

The method measured penicillin’s biological activity rather than relying on a chemical formula, which the Oxford researchers did not yet know. Heatley placed samples of penicillin-bearing liquid in small cylinders set on agar plates containing bacteria. After incubation, the drug produced a clear ring where bacterial growth had been inhibited.

The diameter of that bacteria-free ring provided a practical measure of potency. The Oxford team defined one Oxford unit by the amount needed to produce a 25-millimetre clear zone under specified conditions. This gave researchers a common way to compare batches while they were still working with impure extracts.

The assay was crucial because producing penicillin involved many uncertain steps: growing the mould, filtering the culture liquid, extracting the drug into solvents and recovering it without destroying its activity. Heatley also contributed to the broader production process, including techniques for recovering and concentrating penicillin.

Arthur Gardner studied penicillin’s effects on several bacteria, but he was not the researcher credited with developing this plate assay. Edward Abraham was central to purification and structural work, while Heatley’s assay supplied the measurement needed to guide that work.