Does altered airway mucus composition promote chronic respiratory infections in cystic fibrosis by providing a nutrient source for pathogenic bacteria?

Pseudomonas aeruginosa is an important pathogen in cystic fibrosis (CF) chronically colonising most adult CF patients (McCallum et al, 2001). However, mechanisms by which P. aeruginosa thrives in the CF airway remain unclear because its nutritional requirements are not fully understood. Because mucus is in abundance, a potential nutrient source may come from mucin glycoproteins. Therefore, the purpose of this study was to investigate whether P. aeruginosa could utilise airway mucin as a nutrient source for growth. P. aeruginosa was grown in mucins purified from CF sputum. It was found that P. aeruginosa was unable to utilise mucin, but following DNase treatment growth of P. aeruginosa was significantly promoted. Analysis of gene expression following exposure to DNase treated CF sputum revealed that codA which encodes cytosine deaminase and catalyses the conversion of cytosine to uracil and 5-methylcytosine to thymine was significantly upregulated. The gene, gapA, involved in carbon metabolism was also upregulated. Previous studies have demonstrated that uracil promotes biofilm formation and that uracil and thymine can be metabolised by P. aeruginosa via the reductive pathway of pyrimidine catabolism (Ueda et al, 2009; Kim and West, 1991). Additionally, DNA is in abundance in the CF airway (Lethem et al, 1990). Therefore, it is likely that following dornase alfa treatment, when degraded DNA is liberated it may alter the CF airway mucus composition so that P. aeruginosa growth is promoted. Degraded DNA may have an important role in the pathogenicity of P. aeruginosa and CF disease. This PhD thesis demonstrates novel findings that P. aeruginosa is unable to utilise purified CF patient mucin as a nutrient source for growth, but degraded DNA originating from CF patient sputum can be utilised by P. aeruginosa to produce uracil or thymine which are known to be catabolised via the reductive pathway of pyrimidine catabolism.