COMPARISON OF THE POSTANTIBIOTIC AND POSTANTIBIOTIC SUB-MIC EFFECTS OF LEVOFLOXACIN AND CIPROFLOXACIN ON STAPHYLOCOCCUS-AUREUS AND STREPTOCOCCUS-PNEUMONIAE

The postantiobiotic subminimum inhibitory concentration effect (PA SME ) may simulate in vivo drug exposure more accurately than the postanti biotic effect (PAE) since subinhibitory concentrations of drug persist between antibiotic dosings. In this study, we compared the PAEs and P A SMEs of levofloxacin and ciprofloxacin for clinical isolates of fluo roquinolone-susceptible Staphylococcus aureus and Streptococcus pneumo niae. At two times the MIC, PAEs of levofloxacin were an average of 0. 6 h longer than the PAEs obtained for ciprofloxacin for methicillin-su sceptible and methicillin-resistant S. aureus strains. The PAEs of lev ofloxacin and ciprofloxacin ranged from 1.8 to 3.1 and 1.1 to 2.4 h, r espectively. Continued exposure of the methicillin-resistant strain to 1/16, 1/8, and 1/4 the MIC resulted in PA SMEs of 6.5, 15.3, and >22. 3 h, respectively, for levofloxacin and 3.8, 8.0, and 12.3 h, respecti vely, for ciprofloxacin. For isolates of S. pneumoniae, at two times t he MIC of both fluoroquinolones, the average PAEs of levofloxacin and ciprofloxacin were equivalent: 1.3 h for the penicillin-susceptible is olate and 0.6 h for the penicillin-resistant isolate. Continued exposu re of the penicillin-susceptible S. pneumoniae strain to 1/16, 1/8, an d 1/4 the MIC resulted in average PA SMEs of 1.0, 1.4, and 2.8 h, resp ectively, for levofloxacin and 1.8, 2.0, and 2.5 h, respectively, for ciprofloxacin. Continued exposure of penicillin-resistant S. pneumonia e to 1/16, 1/8, and 1/4 the MIC of the same fluoroquinolones resulted in average PA SMEs of 0.6, 1.1, and 2.9 h, respectively, for levofloxa cin and 0.6, 1.1, and 1.5 h respectively, for ciprofloxacin. The PA SM Es observed demonstrate the superior activity of levofloxacin against methicillin-susceptible or methicillin-resistant S. aureus. Although P AEs were similar for the penicillin-susceptible and pencillin-resistan t S. pneumoniae strains, the PA SME of levofloxacin at one-fourth the MIC was longer for penicillin-resistant S. pneumoniae.