Metabolic and ultrastructural effects induced by ciprofloxacin in Staphylococcus aureus during the postantibiotic effect (PAE) phase.

The new quinolone antibacterial agents have been found to produce a postantibiotic effect (PAE) of 1-4 h in bacteria but the underlying mechanism is still unknown. After exposure to ciprofloxacin in concentrations of 1-16 x MIC for 1 hr the DNA synthesis in S. aureus ATCC 25923 was investigated during the PAE. Ultrastructure was also investigated by electron microscopy after exposure to a concentration of 2 x MIC. The rate of DNA synthesis per organism (measured as CPM/cfu/ml) increased progressively up to a maximum value at 1 1/2 h after drug removal. The maximal 3H-thymidine incorporation per organism at this moment correlated directly with the duration of the PAE. Bacterial swelling with scarcity and vacuole-formation of the cytoplasm characterized the majority of the staphylococci during the PAE. The physiological importance of the increased DNA synthesis is unknown, but the increased metabolic activity, coincident with increased duration of the PAE, suggests a possible common mechanism.