In vitro killing kinetics and postantibiotic effect of josamycin and other macrolides on several bacteria

The antimicrobial activity [minimal inhibitory concentration (MIC) and killing kinetics] and postantibiotic effect (PAE) of different concentrations (MIC and 10 x MIC) of josamycin, erythromycin, midecamycin and azithromycin on Streptococcus pneumoniae, Staphylococcus aureus and Escherichia coli were studied in vitro. The MIC and killing kinetics were determined by standard methods. The PAE was measured after 1 h exposure to the drugs, which were removed by diluting the culture 1,000-fold. All tested macrolides had their lowest MIC against S. pneumoniae, and the most active against S. aureus was josamycin. In killing kinetics, using 10 x MIC, the macrolides exhibited a bactericidal effect against S. pneumoniae and E. coli, with reductions of 3 log10 in colony-forming units per milliliter with respect to the initial inoculum, while a bactericidal effect was not observed for S. aureus. The PAEs were concentration dependent, the greatest PAEs being obtained with S. pneumoniae. Although lower, the PAEs induced on E. coli were significant when high concentrations of the drug were tested. The assayed macrolides showed both more activity and PAEs on S. pneumoniae.