Past exam of the biochemistry course of the University of Cambridge / 2024 / Paper 5 / Section 2 / j / Solution

Several concentrations reveal a sensitivity range and help distinguish resistance from a single-dose threshold effect. Increasing disc concentration increases the region in which diffusing drug exceeds the local growth-inhibitory concentration. The wild type has progressively larger clear zones, act1-117 has little or no detectable inhibition across the tested range, and act1-129 is inhibited much more strongly. The zero-drug disc provides a local control for the disc and vehicle; it is the fourth condition alongside the three nonzero concentrations.

A halo diameter is not directly proportional to a dissociation constant or a minimum inhibitory concentration: molecular diffusion, agar composition, uptake and growth rate contribute. Overlapping large halos can also limit comparison at high doses. Matched lawns, solvent exposure and incubation are necessary, and a quantitative growth assay in uniform drug concentrations would give a cleaner concentration–response curve. Multiple disc doses prevent a lack of inhibition at one weak dose from being mistaken for absolute resistance.