Rates of resistance development to CZA or CT among patients w MDR P. aeruginosa

Among consecutive patients with multidrug-resistant Pseudomonas aeruginosa bacteremia or pneumonia, we found those treated with ceftazidime-avibactam were more likely to develop resistance (defined as >=4-fold increased MIC) than those treated with ceftolozane-tazobactam (40% vs. 10%; P=0.002). Ceftazidime-avibactam resistance was associated with new mutations in ampC and efflux regulatory pathways.