Paradoxical action of penicillin G on Staphylococcus aureus: a time study of the effect of a zonal antibiotic concentration gradient on bacterial growth.

The killing rate of a zonal concentration gradient of penicillin G on Staphylococcus aureus was studied by using a previously described triple-layer technique, enzymatic inactivation of the antibiotic, and an automatic image analyzer. This gradient determines a target image, whose center is the reservoir of antibiotic (constituted by a paper disk containing 100 U of penicillin G) and whose successive concentric zones are: a zone of slow bactericidal activity (corresponding to high levels of penicillin G), a zone of maximal bactericidal activity (corresponding to optimal levels of penicillin G), and a zone of overgrowth situated in front of the antibiotic wave.