Background Ceftolozane–tazobactam (C/T) is a combination of an antipseudomonal cephalosporin with a known β-lactamase inhibitor, with a potent in vitro activity against P. aeruginosa (Pae), without activity against carbapenemases. Among the mechanisms of resistance to C/T that have emerged, substitutions in the Pseudomonal-derived cephalosporinase (PDC), in AmpR, and in some ESBLs, are the most commonly described. The aim of this study was to identify the molecular mechanisms responsible for the in vitro non-susceptibility (NS) to C/T in a group of clinical Pae strains from Latin America.