The invasive properties of Pseudomonas aeruginosa pose a serious threat to the wellbeing of cystic fibrosis (CF) patients; however the specific factors affecting invasiveness are not well understood, especially in chronic infection. This study characterises the invasive profiles of sequential isolates of the same P. aeruginosa strain collected five to eight years apart from five chronically infected adult CF patients. Strains from three patients were characterised as unique isolates and from two patients as the Australian Epidemic strain (AES-1) by pulsed field gel electrophoresis. The capacity of these strains to invade the human alveolar A549 cell line was examined. Later isolates were significantly more invasive than earlier counterparts from the same patient. Quantitative real-time PCR and Western blotting showed that the increase in invasiveness over time was independent of ExoS expression and secretion. A link between clonality and invasiveness was also identified, with AES-1 isolates more invasive than unique isolates. These results suggest that despite a reduction in some virulence factors such as the Type-3 Secretion System (T3SS) during chronic infection, a particular strain can become more invasive over time. Defining mechanisms behind the increased invasiveness during chronic infection may help identify new therapeutic targets for CF patients.
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