Activity of Ceftolozane-Tazobactam against Pseudomonas aeruginosa and Enterobacteriaceae Isolates Collected from Respiratory Tract Specimens of Hospitalized Patients in the United States during 2013 to 2015.

The activities of ceftolozane-tazobactam and comparator agents against organisms deemed to be the cause of pneumonia among patients hospitalized in the United States during 2013 to 2015 were evaluated. Organisms included 1,576 and 2,362 isolates susceptibility tested using reference broth microdilution methods. Ceftolozane-tazobactam, cefepime, ceftazidime, meropenem, and piperacillin-tazobactam inhibited 96.3%, 84.8%, 83.5%, 80.0%, and 78.6%, respectively, of the isolates. Ceftolozane-tazobactam inhibited 77.5 to 85.1% of isolates nonsusceptible to antipseudomonal β-lactams and 86.6% and 71.0% of the 372 (23.6% overall) multidrug- and 155 (9.8%) extensively drug-resistant isolates tested. The activity of this combination was greater than those of other β-lactams evaluated against groups across all U.S. census divisions. Ceftolozane-tazobactam was active against 90.6% of the , being less active than only meropenem (95.6% susceptible) among the β-lactams evaluated. Against 145 and isolates carrying extended-spectrum-β-lactamase (ESBL)-encoding genes without carbapenemases, ceftolozane-tazobactam inhibited 82.8% of these isolates and was more active than cefepime and piperacillin-tazobactam (15.2% and 74.3% susceptible, respectively). ESBL genes included in this analysis were mainly -like (89 isolates) and -like (22) genes but also (31) and (3). Ceftolozane-tazobactam also displayed activity (84.6% susceptible) against 13 isolates harboring acquired AmpC genes. All β-lactams displayed limited activity against -carrying isolates. Ceftolozane-tazobactam was the most active β-lactam tested against isolates from isolates that were the probable cause of pneumonia and displayed activity against , including isolates resistant to cephalosporins and carrying ESBL genes.