Comparison of methods for detection of in vitro synergy in multidrug-resistant Gram-negative bacteria

Abstract

The use of combined antibiotic therapy has become an option for infections caused by multidrug-resistant (MDR) bacteria. The time-kill (TK) assay is considered the gold standard method for the evaluation of in vitro synergy, but it is a time-consuming and expensive method. The purpose of this study was to evaluate two methods for testing in vitro antimicrobial combinations: the disk diffusion method through disk approximation (DA) and the agar gradient diffusion method by MIC:MIC ratio. TK was included as the gold standard. Sixty-two MDR Gram-negative clinical isolates (28 Pseudomonas aeruginosa , 20 Acinetobacter baumannii , and 14 Serratia marcescens ) were submitted to TK, DA, and MIC:MIC ratio synergy methods. Overall, the agreement between the DA and TK assays ranged from 20 to 93%. As the isolates of A. baumannii showed variable results of synergism according to TK, the calculated agreement was statistically significant in this species against fosfomycin with meropenem, including colistin-resistant isolates. The MIC:MIC ratio method showed agreement range from 35 to 71% with TK assays. The kappa test showed a good agreement for the combination of colistin with amikacin (K = 0.58; P = 0.04) among the colistin-resistant A. baumannii isolates. The DA and MIC:MIC ratio methods are easier to perform and might be a more viable tool for clinical microbiology laboratories.