Which organism is most commonly associated with chronic infection in the lungs of individuals with cystic fibrosis?

• A. Staphylococcus aureus
• B. Pseudomonas aeruginosa
• C. Streptococcus pneumoniae
• D. Haemophilus influenzae

Answer: (B)

In cystic fibrosis, the thick, sticky mucus in the airways creates an environment where certain bacteria can persist long-term. Pseudomonas aeruginosa is the organism most commonly linked to chronic lung infection in CF because it readily establishes persistent colonization, especially in its mucoid biofilm–producing form. This biofilm protects the bacteria from antibiotics and the immune system, allowing it to persist for years, drive ongoing airway inflammation, and contribute to progressive lung damage and bronchiectasis.

Staphylococcus aureus often appears early in CF but is less likely to sustain a lifelong chronic infection compared with Pseudomonas. Streptococcus pneumoniae and Haemophilus influenzae are common respiratory pathogens in general and can appear in CF patients, but they do not typically become the dominant chronic colonizers like Pseudomonas aeruginosa.