Effect of sub-inhibitory concentrations of clarithromycin and erythromycin on the production of Staphylococcus aureus capsules.

In order to provide additional data on the in vitro antibacterial activity of cefetamet-pivoxil against respiratory pathogens, we determined the bactericidal kinetics of this antibiotic in comparison to cefixime and ceftibuten against H. influenzae, M. catarrhalis, K pneumoniae, E. coli, S. pneumoniae and S. pyogenes. Time-kill studies were performed by using concentrations equal to 1 x MIC, and 4 x MIC of the antibiotics tested and different inocula (10 5 , 10 7 and 10 9 CFU/ml). The strains were incubated in a shaking incubator at 37°C and 1 ml samples were removed at regular intervals for viable count determination. The antibiotics were eliminated by serial ten-fold dilutions in physiological saline before plating. At 4 x MIC of cefetamet and at 10 5 CFU/ml inoculum the results were 99.9% killing or more of H. influenzae and M. catarrhalis at 4 h, and of E. coli and K. pneumoniae within 4 to 6 h. At the same concentration of MIC and with the same inoculum a 99.9% reduction was achieved against S. pneumoniae within 4 h and against S. pyogenes within 2 h with no regrowth at 24 h in both cases. Similar results were obtained using 10 7 and 10 9 CFU/ml inocula. Cefetamet had efficient killing activity even at lower concentrations. Bactericidal kinetics of cefetamet favorably compare with those of cefixime and ceftibuten. The efficient bactericidal activity of cefetamet-pivoxil indicates a clinical role for this new oral cephalosporin in the treatment of respiratory tract infections.