Iron and Pseudomonas aeruginosa chronic infection

Pseudomonas aeruginosa forms untreatable infections within the hypoxic mucus of the cystic fibrosis (CF) lung. This organism persists because it grows as aggregates of bacteria called biofilms. As iron is essential for bacterial growth, this thesis used in vitro biofilm models to investigate whether targeting iron with chelators can prevent and/or help to eradicate P. aeruginosa biofilm growth and therefore be a potential therapy. Given that P. aeruginosa infections can occur in low-oxygen regions of the CF lung, assays were conducted under both aerobic and anaerobic atmospheres. Previous published studies had focused on the standard laboratory strain PAOl under aerobic conditions. Therefore, the initial part of this thesis characterised biofilm development by clinical CF isolates, strain PAOl and other non-CF isolates under aerobic and anaerobic conditions. The CF isolates overall displayed a reduced ability to form biofilms in short-term biofilm models. In comparison to non-CF isolates, CF isolates also had slower growth rates, exhibited decreased adherence to glass, and decreased motilities (swimming, swarming and twitching). These characteristics were markedly accentuated by anaerobic growth conditions and the findings overall are consistent with the theory that P. aeruginosa exhibits a biofilm growth mode in the CF lung. Moreover, the CF strain phenotypes were not readily reversed by culture manipulations designed to encourage planktonic growth. Incubation of CF isolates in the longer-term continuous-culture flow-through system, where biofilms were followed for one week rather than one day, revealed that they exhibited biofilm differentiation process similar to those reported for strain PAOl. Therefore, results gained from studies of strain PAOl have clinical relevance. Hence, this strain was used in subsequent chapters to investigate the effects of iron on biofilm development. A range of iron-binding compounds impaired biofilm formation under both aerobic and anaerobic atmospheres. Chelators were most effective under anaerobic conditions, ~here they completely prevented biofilm development. This impairment correlated with reducing iron availability. Addition of the iron chelator 2,2' -dipyridyl (2DP) to mature anaerobic biofilm could cause some eradication of the biofilm and also enhance the ability of antibiotic tobramycin to kill bacteria within the biofilm. In conclusion, it is likely that targeting iron will be of clinical benefit in CF individuals. It will be most beneficial if used early and often to prevent the initial growth and transformation of P. aeruginosa into a biofilm.