The most commonly encountered bacterial pathogens in the lungs of patients with cystic fibrosis are:

For reasons that still are not understood, the most common bacterial organisms infecting the lungs of children with cystic fibrosis are Staphylococcus aureus and Pseudomonas aeruginosa. This has not changed much in the past two decades despite major changes in our antibiotic usuage. Pseudomonas is paicularly difficult to eradicate despite sensitivity in vitro to a variety of antibiotics.