In vitro activity of fosfomycin in combination with colistin against clinical isolates of carbapenem-resistant Pseudomas aeruginosa.

The shortage of effective antibiotics against carbapenem-resistant Pseudomonas aeruginosa (CRPA) poses a public health threat. Combination treatment may represent a good choice for treating infections caused by CRPA. The aim of this study was to evaluate the in vitro efficacy of fosfomycin in combination with colistin against clinical CRPA isolates. Eighty-seven isolates were collected from three hospitals in China. The checkerboard method and time-kill assay were used to assess the interactions between fosfomycin and colistin. The fosfomycin/colistin combination displayed synergistic and partial synergistic activity against 21.84% and 27.59% of the isolates, respectively. Antagonism was not observed. In combination, the colistin MIC values were ⩽0.5 μg ml(-1) for 91.95% of the isolates. This result differed significantly from those obtained using a single agent treatment (The colistin MIC values were ⩽0.5 μg ml(-1) for only 25.29% of the isolates). In addition, the time-kill assay demonstrated that the fosfomycin/colistin combination treatment exerted bactericidal effects against five isolates and that the regrowth observed after colistin monotherapy was prevented. In summary, the combination of fosfomycin and colistin demonstrated synergistic activity against the CRPA isolates tested in this study. Furthermore, fosfomycin may potentially widen the therapeutic window of colistin, suggesting that this combination could be applied clinically to control infections caused by CRPA.