Molecular Characterization of Baseline Enterobacterales and Pseudomonas aeruginosa Isolates from a Phase 3 Nosocomial Pneumonia (ASPECT-NP) Clinical Trial

We reviewed beta-lactam-resistant baseline Enterobacterales species and Pseudomonas aeruginosa lower respiratory tract isolates collected during the ASPECT-NP phase 3 clinical trial that evaluated the safety and efficacy of ceftolozane-tazobactam compared with meropenem for the treatment of nosocomial pneumonia in ventilated adults. Isolates were subjected to whole-genome sequencing, real-time PCR for the quantification of the expression levels of beta-lactamase and efflux pump genes, and Western blot analysis for the detection of OprD (P. aeruginosa only). Extended-spectrum beta-lactamase (ESBL) genes were detected in 168 of 262 Enterobacterales isolates, and among these, bla(CTX-M-15) was the most common, detected in 125 isolates. Sixty-one Enterobacterales isolates carried genes encoding carbapenemases, while 33 isolates did not carry ESBLs or carbapenemases. Carbapenemase-producing isolates carried mainly NDM and OXA-48 variants, with ceftolozane-tazobactam MIC values ranging from 4 to 128 mu g/ml. Most ceftolozane-tazobactam-nonsusceptible Enterobacterales isolates that did not carry carbapenemases were Klebsiella pneumoniae isolates that exhibited disrupted OmpK35, specific mutations in OmpK36, and, in some isolates, elevated expression of bla(CTX-M-15). Among 89 P. aeruginosa isolates, carbapenemases and ESBL-encoding genes were observed in 12 and 22 isolates, respectively. P. aeruginosa isolates without acquired beta-lactamases displaying elevated expression of AmpC (14 isolates), elevated expression of efflux pumps (11 isolates), and/or a decrease or loss of OprD (22 isolates) were susceptible to ceftolozane-tazobactam. Ceftolozane-tazobactam was active against >75% of the Enterobacterales isolates from the ASPECT-NP trial that did not carry carbapenemases. K pneumoniae strains resistant to ceftolozane-tazobactam might represent a challenge for treatment due to their multiple resistance mechanisms. Ceftolozane-tazobactam was among the agents that displayed the greatest activity against P. aeruginosa isolates.