Ceftolozane/Tazobactam Resistance and Mechanisms in Carbapenem-Nonsusceptible Pseudomonas aeruginosa

Abstract

This study established the in vitro activity of ceftolozane/tazobactam (C/T) and its genotypic resistance mechanisms by whole-genome sequencing (WGS) in 195 carbapenem-nonsusceptible Pseudomonas aeruginosa (CNSPA) clinical isolates recovered from Singapore between 2009 and 2020. C/T susceptibility rates were low, at 37.9%. Cross-resistance to ceftazidime/avibactam was observed, although susceptibility to the agent was slightly higher, at 41.0%. Whole-genome sequencing revealed that C/T resistance was largely mediated by the presence of horizontally acquired β-lactamases, especially metallo-β-lactamases. These were primarily disseminated in well-recognized high-risk clones belonging to sequence types (ST) 235, 308, and 179. C/T resistance was also observed in several non-carbapenemase-producing isolates, in which resistance was likely mediated by β-lactamases and, to a smaller extent, mutations in AmpC-related genes. There was no obvious mechanism of resistance observed in five isolates. The high C/T resistance highlights the limited utility of the agent as an empirical agent in our setting. Knowledge of local molecular epidemiology is crucial in determining the potential of therapy with novel agents.IMPORTANCEPseudomonas aeruginosa infection is one of the most difficult health care-associated infections to treat due to the ability of the organism to acquire a multitude of resistance mechanisms and express the multidrug resistance phenotype. Ceftolozane/tazobactam (C/T), a novel β-lactam/β-lactamase inhibitor combination, addresses an unmet medical need in patients with these multidrug-resistant P. aeruginosa infections. Our findings demonstrate geographical variation in C/T susceptibility owing to the distinct local molecular epidemiology. This study adds on to the growing knowledge of C/T resistance, particularly mutational resistance, and will aid in the design of future β-lactams and β-lactamase inhibitors. WGS proved to be a useful tool to understand the P. aeruginosa resistome and its contribution to emerging resistance in novel antimicrobial agents.

Keywords: Pseudomonas aeruginosa; ceftolozane/tazobactam; molecular characterization.

FIG 1

Mechanisms of ceftolozane/tazobactam (C/T) resistance in 86 carbapenemase-producing CNSPA isolates. a, Bold values indicate ceftazidime/avibactam (CZA) susceptibility. b, The main chromosomal mutations (ampC, ampR, dacB, and ftsI) leading to amino acid substitutions compared to the reference wild-type comparator amino acid sequences from Pseudomonas aeruginosa PAO1 are shown. The list of nonsynonymous variations were refined to include only those more likely to be involved in the C/T-resistant phenotype, i.e., (i) mutations with known effect on resistance according to published evidence and (ii) mutations with predicted functional impact (i.e., deleterious) and not identified in wild-type/susceptible isolates. There were no mutations found in ampR in this set of isolates. PDC, Pseudomonas-derived cephalosporinase; ST, sequence type.

FIG 2

Mechanisms of C/T resistance in 35 non-carbapenemase-producing CNSPA isolates. a, Bold values indicate CZA susceptibility. b, The main chromosomal mutations (ampC, ampR, dacB, and ftsI) leading to amino acid substitutions compared to the reference wild-type comparator amino acid sequences from Pseudomonas aeruginosa PAO1 are shown. The list of nonsynonymous variations was refined to include only those more likely to be involved in the C/T-resistant phenotype, i.e., (i) mutations with known effect on resistance according to published evidence and (ii) mutations with predicted functional impact (i.e., deleterious) and not identified in wild-type/susceptible isolates. *, G439C amino acid substitution in OXA-10; ^{^}, A163T amino acid substitution in AmpC; ⁺, ΔK74-E75 in AmpC; ^#, frameshift (FS) at position 149.