A critical comment on the use of the plate diffusion test for QSAR considerations

The plate diffusion test is widely used for the screening of possible antimicrobials. Antibiograms play an important role for the selection of suitable antibiotics in the routine treatment of infections. We are able to show with a series of O-acylated 4-hydroxybenzoic acids that the percentage of the dose which did not diffuse into the agar plate depends on physico-chemical parameters of the compounds. The amount of the residue is linearly related both to the lipophilic behaviour expressed by the partition coefficients (log Pf) and the spatial extension expressed as Van der Waals Volumes (Vw). The plate diffusion test should be used with caution for studies of QSAR and also relative comparisons of activities.