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Abstract
Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen that can cause acute infections such as keratitis, burn wound infections and ventilator associated pneumonia. P. aeruginosa is also the main cause of chronic infections in cystic fibrosis (CF) patients, and presents symptoms such as acute exacerbations and loss of respiratory function due to damage of lung tissue and ultimately, death.
In environments such as the CF lung, P. aeruginosa is known to form biofilms in order to colonise, infect and persist in its host. P. aeruginosa biofilms are thought to be important in protecting bacteria from various environmental stresses such as protozoan grazing, surfactant stress, antibiotic stress, toxins, and the host immune response. During the cell death and dispersal phase, a bacteriophage known as Pf4 has been isolated from P. aeruginosa biofilms. Pf4 is a filamentous prophage that contributes to the stability of the developing biofilm and virulence of P. aeruginosa, but it is not clear how this occurs. The main aim of this study was to determine the role of the Pf4 phage in P. aeruginosa biofilm resistance against stress, and virulence. This included further investigating the effect of Pf4 on the stability of P. aeruginosa biofilms under stress, as well as studying how the Pf4 phage influences selected P. aeruginosa virulence phenotypes.
Co-incubation experiments showed that the wild-type P. aeruginosa outcompeted the Pf4 mutant deletion strain, further supporting the hypothesis that the Pf4 phage is selected for and advantageous for P. aeruginosa. Experiments assessing virulence phenotypes showed that the Pf4 phage influences the invasiveness and cytotoxic capability of P. aeruginosa in varying ways depending on the mammalian cell type, but does not contribute to the virulence of P. aeruginosa towards C. elegans. The presence of the Pf4 phage also negatively regulates the production of the pyoverdine siderophore, as determined through analysis of planktonic cultures, and determining gene expression in planktonic and late biofilm cultures. In regards to understanding the effect of the Pf4 phage on P. aeruginosa biofilm stability under stress, it was observed that the phage increased resistance of P. aeruginosa biofilms to ciprofloxacin (fluoroquinolone) and paraquat (superoxide) stress.