Introduction: Antibiotics for treating infectious diseases caused by carbapenem-resistant Gram-negative pathogens (CR-GNOs) are very limited in clinical practice. We aim to provide supportive evidence by revealing the combined effect of aztreonam (ATM) and amoxicillin/clavulanic acid (AMC) against GNOs with carbapenem resistance mediated by metallo-β-lactamase (MBL).
Methods: All isolates were identified by the VITEK system and EDTA inhibitory assays. PCR followed by sequencing was conducted to confirm the genotypes of MBL and extended spectrum β-lactamase (ESBL). Time kill assay was performed to clarify the bactericidal effect of drug combination.
Results: A total of 59 MBL-producing CR-GNOs (33 Enterobacteriaceae  spp. isolates and 26 Pseudomonadales isolates) were identified and there found three MBL genes, namely, bla _IMP, bla _NDM and bla _VIM, with ratios of 76.2%, 11.8% and 11.8%, respectively. The Enterobacteriaceae  spp. isolates were commonly positive for the ESBL genes, including bla _TEM (18 isolates), bla _SHV (20 isolates) and bla _CTX-M-1 (8 isolates), while the P. aeruginosa  isolates were positive for bla _OXA-10 (11 isolates). The checkerboard microdilution assay was used to detect combination effect of ATM and AMC, which showed synergy (97.0%) and partial synergy (3.0%) in Enterobacteriaceae  spp. isolates, and partial synergy (42.3%) and indifference (34.6%) in the Pseudomonadales isolates. Four Enterobacteriaceae  spp. isolates were selected for a time-kill assay, and rapid bactericidal effects were observed in the combination groups compared to the control and mono-ATM groups; these effects began in the first hour and continued to the sixth hour, yielding a 5- to 7-fold reduction in Log10 CFU/mL.
Discussion: The combination of ATM and AMC would be an available option to control infections caused by MBL-producing CR-GNOs, especially Enterobacteriaceae  spp. isolates that coproduce ESBLs, and exhibit significant synergic effects in vitro.
Keywords: gram-negative pathogens, carbapenem resistance, metallo-β-lactamase, extended spectrum β-lactamase, aztreonam, amoxicillin/clavulanic acid, combination