Standard vs. Biofilm Susceptibility Testing in Cystic Fibrosis (CF)

Inclusion Criteria: * Diagnosis of CF based on the following: sweat chloride \> 60 mEq/L (by quantitative pilocarpine iontophoresis), or genotype with 2 identifiable mutations consistent with CF; and one or more clinical features consistent with CF. * Age ≥ 14 years (changed from ≥ 18 years by protocol amendment). * Able to expectorate sputum at screening. * History of persistent positivity for P. aeruginosa on respiratory culture (at least three positive oropharyngeal (OP), sputum and/or bronchoscopy cultures in the 24 months prior to screening). * Able to reproducibly perform pulmonary function testing. * Clinically stable at screening, with no evidence of pulmonary exacerbation. * Written informed consent provided. Exclusion Criteria: * Sputum culture negative for P. aeruginosa or density less than 10E5 CFU/gm at screening. * Sputum culture positive for B. cepacia at screening. * Presence of P. aeruginosa in sputum with off-scale resistance to all antibiotics by either method of susceptibility testing at screening. (changed from multiply-resistant P. aeruginosa by protocol amendment) * History of B. cepacia positive respiratory culture within 24 months prior to screening. * Hospitalization or treatment for a pulmonary exacerbation within 2 months prior to screening. * Administration of parenteral anti-pseudomonal antibiotics within 2 months prior to screening. * Treatment with oral or inhaled anti-pseudomonal antibiotics, or azithromycin or other macrolides within 14 days prior to screening. * History of allergy (urticarial rash, diffuse erythroderma, serum sickness) to more than two groups of antibiotics (aminoglycosides, penicillins, cephalosporins, monobactams, macrolides, or quinolones) that are a therapeutic option. * History of anaphylaxis or other life threatening complication to any antibiotic in the six groups that are a therapeutic option. * History of abnormal renal function (serum creatinine \> 1.5 x upper limit of normal) within one year of enrollment. * History of abnormal liver function tests (\> 2.5 x upper limit of normal) within one year of enrollment. * Clinically documented hearing loss that precludes treatment with aminoglycosides. * Post lung transplantation. * Positive pregnancy test or female who is lactating or is not practicing an acceptable method of birth control. * Presence of a condition or abnormality that in the opinion of an investigator would compromise the safety of the patient or the quality of the data. * Administration of any investigational agent within 30 days prior to screening.