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Paradoxical effect of cloxacillin and benzylpenicillin against clinical isolates in Staphylococcus aureus
1Department of Infectious Diseases and Clinical Bacteriology, Akademiska Sjukhuset, Uppsala University, Sweden.
Abstract:
The occurrence of a paradoxical effect of cloxacillin and benzylpenicillin in 7 strains of Staphylococcus aureus was studied with time killing curves in vitro. The isolates were exposed to 1, 2, 10 and 100 x MIC of the antibiotics. A paradoxical effect was found in 5/7 strains with cloxacillin and in 4/7 with benzylpenicillin. All strains were killed faster with 2 x MIC than with 100 x MIC. All strains that showed a paradoxical effect with cloxacillin did so already at 10 x MIC. For benzylpenicillin the effect occurred at 10 x MIC for 2 strains and at 100 x MIC for 2. A single MBC/MIC ratio will not reveal the paradoxical effect.