Bactericidal activity of cefepime and ceftriaxone tested against Streptococcus pneumoniae

The bactericidal activities of cefepime and ceftriaxone were assessed by testing a contemporary collection of 50 Streptococcus pneumoniae strains. Minimum inhibitory and bactericidal concentrations (MIC and MBC, respectively) of cefepime and ceftriaxone were determined, and time-kill studies were performed on 14 selected strains (10 penicillin-resistant, 2-intermediate, and 2-susceptible). Cefepime and ceftriaxone showed essentially identical potency (MIC50, 1 microg/mL and MIC90, 2 microg/mL, for both compounds) and MBC values (MBC50, 1 microg/mL for both). MBC/MIC ratios were < or = 4 for cefepime and < or = 8 for ceftriaxone on 48 (96.0%) strains, and 2 strains (4.0%) displayed MBC/MIC ratios > or = 32 (tolerance) to the 2 cephalosporins. Time-kill curves corroborated the MBC/MIC studies. Cefepime and ceftriaxone bactericidal activity (> or = 3 log10 CFU/mL reduction in inoculum) was demonstrable after 24 h of exposure to 8x MIC for 13 (92.9%) of 14 strains, whereas 1 strain showed approximately 2 log10 CFU/mL reduction. In conclusion, our results indicate that cefepime and ceftriaxone exhibit comparable potency and bactericidal activities when tested against contemporary pneumococcal strains with varying penicillin susceptibility patterns. Both parenteral cephems offer alternative therapeutic choices for the treatment of invasive pneumococcal infections.