= Solution
\b[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.
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