Comparison of synergism between colistin, fosfomycin and tigecycline against extended-spectrum β-lactamase-producing Klebsiella pneumoniae isolates or with carbapenem resistance

Purpose: To investigate the synergistic and bactericidal effects of antimicrobial combinations of any two of colistin, fosfomycin and tigecycline against the nine extended-spectrum β-lactamase (ESBL)-producing Klebsiella pneumoniae (KP) clinical isolates, including 4 carbapenem-susceptible strains and five imipenem and/or meropenem-resistant strains. Methods: In vitro synergism and bactericidal activity of combination of colistin, fosfomycin and tigecycline were evaluated by time-kill studies in standard inoculum of bacterial densities of a suspension containing 5 Ã 105 CFU/mL by using 1/2à MIC for each alone, and both 1/2à and 1/4à MIC for any two drugs. The settings of low MIC dosing were allowed to rapidly survey the most active drug combination. Results: The most active combination group was colistin plus tigecycline, showing synergy in 8 isolates and bactericidal activities in 6 isolates by using concentrations of 1/2à MIC and 1/4à MIC, respectively. The least active combination was tigecycline plus fosfomycin, which showed synergy in only 4 isolates and no bactericidal activities by using concentrations of 1/2à MIC and 1/4à MIC, respectively. Conclusions: The combination of tigecycline and colistin may be considered as a last-resort approach to the ESBL-producing KP infections, especially those isolates with carbapenem resistance. Keywords: Carbapenem resistance, Colistin, ESBL, Fosfomycin, Tigecycline