 Participants will receive 4-6 doses of ceftolozanetazobactam 3 grams every 8 hours, in addition to standard intravenous antibiotic therapy selected by the site. Just prior and then after the final dose, a total of six blood samples will be collected to measure ceftolozane and tazobactam concentrations. Data will be fit to a population pharmacokinetic model. The final model will be utilized in a Monte Carlo simulation to determine the probability of several different dosing regimens retaining concentrations above the minimum inhibitory concentration MIC for at least 39 of the dosing interval. These data will be utilized to determine an optimized dosing regimen for adults with CF. 