Microbial and host innate immune factors affecting the persistence of Staphylococcus aureus in cystic fibrosis airways.

Chronic bacterial respiratory infections by Staphylococcus aureus are a hallmark of cystic fibrosis (CF) lung disease, affecting up to 80% of all people with CF by their mid-teens. S. aureus is able to survive and persist in the CF lung despite robust neutrophilic inflammation. As neutrophils are the immune system's front line of defense against bacteria, the persistent nature of S. aureus infections in CF indicates both a defect in the ability of neutrophils to kill S. aureus and an enhanced ability of the bacteria to survive. S. aureus persistence in the CF lung is driven by both microbial and host, genetic and microenvironmental, factors. There doesn't seem to be one unifying feature that makes S. aureus more virulent in the CF lung as several factors have been proposed to aide its survival. These include increased resistance to antibiotics, the ability to form small colony variants, biofilm formation, co-infection with Pseudomonas aeruginosa, and several virulence factors such as the accessory gene regulatory system, leukocidins, and staphylococcal protein A. A variety of host factors also affect the ability of neutrophils to kill S. aureus in the CF lung. Defects in the function of the cystic fibrosis transmembrane conductance regulator, the genetic cause of CF, affect phagolysosomal killing and lead to increased formation of neutrophil extracellular traps, which are less effective at killing S. aureus. Additionally, data suggest that factors within the CF lung microenvironment also affect neutrophilic killing of S. aureus. However, more research is needed to clearly identify what these environmental factors may be. This review article summarizes the current knowledge on the clinical relevance of S. aureus lung infections in CF, on microbial and host mechanisms promoting S. aureus survival in the CF lung, and on details of neutrophil-S. aureus interactions in CF. By understanding how S. aureus is able to survive in the CF lung and why neutrophils are unable to kill this bacterium, it could be possible to identify potential therapeutic targets to alleviate the consequences of S. aureus respiratory infection in CF.