In vitro activity of ceftolozane-tazobactam as determined by broth dilution and agar diffusion assays against recent U.S. Escherichia coli isolates from 2010 to 2011 carrying CTX-M-type extended-spectrum β-lactamases

Ceftolozane MIC(50)/MIC(90)s were 4/8 μg/ml when tested against 26 CTX-M-14-type-producing isolates and 64/>64 μg/ml against 219 CTX-M-15-type-producing isolates. The addition of 4 μg/ml tazobactam lowered the ceftolozane MIC50/MIC9(0)s to ≤ 0.25/0.5 μg/ml by broth microdilution and Etest. The zone diameters for the ceftolozane-tazobactam disks were 23 to 29 mm for 92.2% of the isolates.