Antimicrobial Activity Study Of Fosfomycin Combined With Tigecycline Against KPC-producing Klebsiella Pneumoniae

Objectives:Evaluate antimicrobial activity of fosfomycin combined with tigecycline against KPC-producing Klebsiella pneumoniae and study the mechanism of fosfomycin resistance.Methods:Broth microdilution method was used to determine the minimum inhibitory concentrations (MICs) of fosfomycin and tigecycline against42Klebsiella pneumoniae isolates (including20KPC-producing and22KPC non-producing isolates). Checkerboard design method was applied to evaluate different concentrations of antimicrobial susceptibility combined effect and calculate the fractional inhibitory Concentration (FIC) index (FICI). FICI=MlCfosfomycin joint/MICfosfomycin monotherapy+MIC tigecycline joint/MIC tigecycline monotherapy Related interpretation criteria are as follows:FICI≤0.5means synergy,0.5<FICI≤1.0means additive,1<FICI≤2means autonomy, and FICI>2means antagonism. In addition, this study also screened for fosfomycin resistant genes in KPC producing Klebsiella pneumoniae isolates and pulsed-field gel electrophoresis (PFGE) and Southern hybridization experiments were used to determine the location of fosfomycin resistance gene fosA.Results:Antimicrobial susceptibility testing results indicated that the fosfomycin and tigecycline susceptibility rates were35%(7/20) and70%(14/20) respectively in KPC-producing group. After combination therapy, the susceptibility rates were increased to50%(10/20) and95%(19/20), and both MICs were decreased. MIC of tigecycline decreased significantly after combination therapy and showed a statistical difference (P=0.013) compared with the MIC of monotherapy; whereas there was no significant difference (P=0.211) to fosfomycin. FIC index indicated that a total of60%isolates showed synergy and additive effects between two antimicrobial agents, followed by autonomy, but there was no antagonism effects. In addition, the fosfomycin and tigecycline susceptibility rates were68.2%(15/22) and69.1%(13/22) respectively in KPC non-producing group. After combination therapy, the susceptibility rates were increased to68.2%(15/22) and86.4%(19/22), and both MICs were decreased except for MIC90of fosfomycin. FIC index indicated that the main effects between two antimicrobial agents was autonomy, followed by additive, synergy, but without antagonism. Moreover, this study also screened for fosfomycin resistant genes in13KPC producing Klebsiella pneumoniae isolates and identified two fosA-producing isolates. Pulsed-field gel electrophoresis (PFGE) and Southern hybridization experiments indicated that these two fosA-producing isolates were located on the same plasmid as KPC gene, and its size were138.9kb and104.5kb respectively.Conclusions:KPC-producing Klebsiella pneumoniae showed more sensitive to tigecycline and about half to fosfomycin. A total of60%isolates showed synergy and additive effects between two antimicrobial agents, while the latter accounted for major part, followed by autonomy, but without antagonism. These findings could be adopted as clinical therapy guidance. In addition, this study also found KPC-producing Klebsiella pneumoniae plasmid carrying fos/A gene, which lead to fosfomycin resistance.