To evaluate the activity of meropenem–amikacin and meropenem–colistin combinations with checkerboard broth microdilution (CKBM) compared with isothermal microcalorimetry (ITMC) assays against a multi-centric collection of multi-drug-resistant Gram-negative clinical isolates; and to compare the fractional inhibitory concentration (FIC) index and time to results of CKBM and ITMC.

A collection of 333 multi-drug-resistant Gram-negative clinical isolates showing reduced susceptibility to meropenem (121 Klebsiella pneumoniae, 14 Escherichia coli, 130 Pseudomonas aeruginosa and 68 Acinetobacter baumannii) isolated from different centres (Florence, Madrid, Rotterdam and Stockholm) was included in the study. The antimicrobial activity of meropenem–amikacin and meropenem–colistin combinations was evaluated with CKBM and ITMC. FIC index results were interpreted as synergistic/additive and indifferent for values ≤0.5/0.5<x≤1 and >1, respectively. Whole-genome sequencing data of a subset of strains were used to evaluate their clonality.

In total, 254 and 286 strains were tested with meropenem–colistin and meropenem–amikacin combinations with ITMC and CKBM, respectively. Synergistic/additive effects were observed for 46 strains (20 K. pneumoniae, four E. coli, 22 P. aeruginosa) and 20 strains (three K. pneumoniae, 11 P. aeruginosa and six A. baumannii) with meropenem–amikacin and meropenem–colistin combinations, respectively, with CKBM. ITMC showed good concordance with CKBM, with 89.5% and 92.2% of cases interpreted within the same FIC index category for meropenem–amikacin and meropenem–colistin combinations, respectively. Most of the synergistic/additive effects were detected within 6 h by ITMC.

ITMC showed very good concordance with CKBM against a large collection of multi-drug-resistant Gram-negative clinical isolates, and could be implemented for the rapid evaluation of in-vitro activity of antimicrobial combinations.

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