Postexposure factors influencing the duration of postantibiotic effect: significance of temperature, pH, cations, and oxygen tension

Abstract

The purpose of the present study was to assess and compare the impacts of various postexposure conditions on postantibiotic effect (PAE). PAEs were induced in Staphylococcus aureus and Escherichia coli by exposing the organisms to different antibiotics (penicillin G, ampicillin, erythromycin, ciprofloxacin, and gentamicin) at 5 or 10 times the MIC in plain Mueller-Hinton broth for 1 h at 35 degrees C. Regrowth was determined by measuring the viable counts after drug removal by a 10(-3) or 10(-4) dilution procedure under various postexposure conditions (incubation temperatures at 20, 25, 30, or 35 degrees C; growth under shaken, unshaken, anaerobic conditions; pH 6.0, 7.4, or 9.0; and with sodium chloride concentrations at 0, 1, 3, or 6%). PAE increased in response to a decrease in incubation temperature from 35 to 20 degrees C, and a significant correlation between bacterial generation times and duration of PAEs (r2, 0.82 to 0.97) was demonstrated. The duration of PAE was also modified by the pH in the regrowth medium. PAE increased considerably for S. aureus at pH 6.0 and 9.0 compared to that at pH 7.4 after induction with penicillin G, and with gentamicin the PAE against S. aureus recovering at pH 6.0 also increased considerably. A high concentration of sodium chloride in the regrowth medium produced the most extensive changes in PAE except for that against E. coli induced by ampicillin. PAE increased significantly in response to increased salinity. No recovery even after overnight incubation was detected for S. aureus after preexposure to penicillin, ciprofloxacin, or gentamicin. Only minor changes in the duration of PAE were observed in relation to recovery oxygen tension. It is concluded that many postexposure factors have a profound effect on the duration of PAE.