Success of ceftazidime-avibactam and aztreonam in combination for a refractory biliary infection with recurrent bacteraemia due to blaIMP-4 carbapenemase-producing Enterobacter hormaechei subsp. oharae.

While aztreonam inhibits MBLs, MBL-positive isolates often express other beta-lactamase enzymes. Avibactam inhibits ESBLs and other beta-lactamases, and its use in this case possibly contributed to therapeutic success due to inhibition of the concomitant blaTEM-4 in the isolates. This case demonstrates that phenotypic antimicrobial susceptibility testing (layered Etests for synergy), backed up by WGS, can produce results that allow tailored antimicrobial therapy in difficult infections. This case adds to the evidence for using CAZ-AVI-AZT in serious MBL infections.