The purpose of this study is to look at pulmonary exacerbations in people with cystic fibrosis (CF) that need to be treated with antibiotics given through a tube inserted into a vein (intravenous or IV). A pulmonary exacerbation is a worsening of respiratory symptoms in people with CF that needs medical intervention. Both doctors and CF patients are trying to understand the best way to treat pulmonary exacerbations. This study is trying to answer the following questions about treating a pulmonary exacerbation: * Do participants have the same improvement in lung function and symptoms if they are treated with one type of antibiotic (called beta-lactams or β-lactams) versus taking two different types of antibiotics (tobramycin and β-lactams)? * Is taking one type of antibiotic just as good as taking two types?