Multidrug‑resistant Acinetobacter baumannii strains with NDM‑1: Molecular characterization and in vitro efficacy of meropenem‑based combinations

Acinetobacter baumannii is an important cause of hospital‑acquired, multidrug‑resistant (MDR) infections occurring worldwide. Anti‑microbial combination regimens may be the only feasible treatment option for affected patients. In the present study, the efficacy of the combined therapy of meropenem with colistin, ampicillin‑sulbactam, tazobactam and vancomycin against clinical strains of MDR A. baumannii was determined. Anti‑microbial susceptibility testing was performed and resistance genes were characterized by a multiplex polymerase chain reaction (PCR)‑reverse line blot assay. The genetic background of New Delhi metallo‑β‑lactamase 1 (NDM‑1) was analysed by primer walking. The presence of NDM‑1 was detected using the modified Hodge test and the EDTA‑combined disk test. To screen for synergistic drug effects, the fractional inhibitory concentration index was calculated using a checkerboard assay. The results of the PCR as well as the sequence analyses suggested that NDM‑1 was located downstream of the ISAba125 element. In addition, a synergistic effect was determined for meropenem + vancomycin, meropenem + tazobactam and meropenem + ampicillin + sulbactam in two strains each, and in four strains for meropenem + colistin. A total of five A. baumannii strains with resistance to numerous antibiotics and carrying numerous resistance genes were identified. In the strains of A. baumannii, the NDM‑1 gene was integrated in a transposon structure with a copy of the ISAba125 insertion sequence. However, the genetic background was not identical among the different species and strains. The genetic variability of NDM‑1 may facilitate the rapid dissemination of this gene. In conclusion, meropenem may enhance the efficacy of antibiotics in A. baumannii strains with NDM‑1‑associated MDR.