1587. Comparative In Vitro Antipseudomonal Activity of Ceftolozane/Tazobactam Against Pseudomonas aeruginosa Isolates from Children with Cystic Fibrosis

BACKGROUND: Ceftolozane/tazobactam (C/T) is a relatively new antipseudomonal cephalosporin combined with a β-lactamase inhibitor approved by the FDA in 2014. The study goal was to evaluate its in vitro activity vs. comparator agents against a pre-selected panel of Pseudomonas isolates obtained from pediatric patients with cystic fibrosis. METHODS: Clinical Pseudomonas isolates from 2 free-standing pediatric centers were obtained from respiratory samples from patients with cystic fibrosis during 2015–2017. Stored isolates were cultured on blood agar (Thermo Fisher Scientific) at 35±1°C for 18–24 hours. A 0.5 McFarland suspension was prepared with Sensititre® demineralized water. Final inocula of 5 × 10E(5) CFU/mL were prepared in Sensititre® Mueller–Hinton broth. Custom-prepared Sensititre® MIC plates (Thermo Fisher Scientific) containing C/T and 10 comparator antimicrobials were inoculated and incubated at 35±1°C for 18–24 hours. MICs were determined via Sensititre Vizion® system. MIC endpoints (susceptibilities) were interpreted by CLSI (2018) breakpoint criteria. RESULTS: Data from 83 unique isolates from 2 sites (Missouri: 38 and Texas: 45) for the years 2015–2017 are reported. Overall, 90% of the tested isolates were C/T susceptible (MIC ≤4 μg/mL), while susceptibility for colistin, meropenem, and ciprofloxacin were 93%, 88%, and 86%, respectively (Table 1). C/T exhibited high overall activity (MIC50/90, 1/4 μg/mL) against these Pseudomonas isolates. C/T was more active than amikacin, aztreonam, cefepime, ceftazidime, ciprofloxacin, gentamycin, meropenem, piperacillin–tazobactam and tobramycin against tested Pseudomonas isolates but less active than colistin. CONCLUSION: C/T had broad-spectrum activity and high potency against most Pseudomonas aeruginosa from 2 geographically diverse pediatric US medical centers. [Image: see text] DISCLOSURES: All authors: No reported disclosures.