ERR subjects [≥8% predicted improvement in FEV1 from Visit 1 to Visit 2 and CRISS reduction of ≥11 points from Visit 1 to Visit 2] will be randomized 1:1 to either 10 days or 14 days total IV antibiotic treatment duration. Remaining (NERR) subjects will be randomized 1:1 to receive either 14 or 21 days total IV antibiotic treatment duration. All subjects will be evaluated again at Visit 3, 14 days following scheduled completion of IV antibiotic treatment.