Study of In Vitro Synergistic Bactericidal Activity of Dual β-Lactam Antibiotics Against KPC-2-Producing Klebsiella pneumoniae

Objectives: To study the in vitro synergistic bactericidal activity of dual β-lactam antibiotics against KPC-2-producing Klebsiella pneumoniae and to explore the new therapeutic regimens for infections caused by carbapenem-resistant strains. Materials and Methods: The antimicrobial susceptibility testing of imipenem, meropenem, ceftazidime, and clavulanic acid on 40 clinically isolated strains of KPC-2-producing K. pneumoniae from 5 cities across the country was performed by microdilution broth method. The in vitro synergistic bactericidal activity of combined antibiotics mentioned above was determined at various concentrations using checkerboard techniques. The combination of antibiotics include imipenem with clavulanic acid, meropenem with clavulanic acid, imipenem with ceftazidime, meropenem with ceftazidime, and meropenem with imipenem. The combined effectiveness of synergistic, indifferent, or antagonistic was calculated by fractional inhibitory concentration indexes. Based on the results of synergistic bactericidal activity, 16 strains were selected for time-kill assays. Results: All 40 strains of K. pneumoniae were shown resistant to every single antimicrobial agent tested, with minimal inhibitory concentrations of carbapenems >32 mg/L in most isolates. None of the combinations was antagonistic. Synergies of combination of imipenem with clavulanic acid, or imipenem with ceftazidime were observed in 80% (32/40) and 7.5% (3/40) of strains, respectively; Combinations of meropenem and clavulanic acid, or meropenem and ceftazidime revealed a synergistic antibacterial activity on 25% (10/40) and 30% (12/40) of strains, respectively. Synergy of meropenem and imipenem combination was shown in 30% (12/40) of strains. Time-kill assays validated the data from checkerboard testing. Conclusions: The study strongly supported the hypothesis that combined dual β-lactam antibiotics might be effective in the treatment of infections caused by KPC-2-producing K. pneumoniae. The combination of imipenem and clavulanic acid possessed the best efficiency, followed by the regimens of combined meropenem-ceftazidime and imipenem-meropenem.