In vitro evaluation of the activity of β-lactams,new quinolones,and other antimicrobial agents againstStreptococcus pneumoniae

The in vitro activities of the β-lactam and quinolone antibiotics panipenem, cefpodoxime, cefdinir, cefditoren, faropenem, tosufloxacin, levofloxacin and grepafloxacin were compared with similar conventional antibiotics against penicillin-resistantStreptococcus pneumoniae (PRSP). Pneumococcal isolates collected from October 1994 to March 1995 (n=1283) consisted of penicillin-susceptibleS. pneumoniae (PSSP; 59.2%), penicillin-intermediately-resistantS. pneumoniae (PISP;11.2%), and PRSP (29.6%). The isolates were highly susceptible to panipenem, faropenem and cefditoren with MIC₉₀ values of 0.125 μg/mL, 0.5 μg/mL and 0.5 μg/mL, respectively. Correlation coefficients for the relationships between the MICs of these β-lactam agents and that of penicillin G ranged from γ=0.7652 to γ=0.8022. These new β-lactam agents produced excellent bactericidal responses at concentrations greater than their MICs for PSSP concomitant with appropriate cellular morphologic changes. However, the bactericidal action of these antibiotics against PRSP was less pronounced and fewer instances of cell lysis were observed. The MIC₉₀ of cefpodoxime was similar to that of cefaclor, whereas that of cefdinir was between those of faropenem and cefpodoxime. The MIC distribution of the new quinolone agents showed 1 peak, but the MIC₉₀ values of tosufloxacin and grepafloxacin were both 0.5 μg/mL and that of levofloxacin was 2.0 μg/mL. Only 1% of all isolates demonstrated cross-resistance to all quinolone agents.