Differential infection properties of three inducible prophages from an epidemic strain of Pseudomonas aeruginosa

Background: Pseudomonas aeruginosa is the most common bacterial pathogen infecting the lungs of patients with cystic fibrosis (CF). The Liverpool Epidemic Strain (LES) is transmissible, capable of superseding other P. aeruginosa populations and is associated with increased morbidity. Previously, multiple inducible prophages have been found to coexist in the LES chromosome and to constitute a major component of the accessory genome not found in other sequenced P. aerugionosa strains. LES phages confer a competitive advantage in a rat model of chronic lung infection and may, therefore underpin LES prevalence. Here the infective properties of three LES phages were characterised. Results: This study focuses on three of the five active prophages (LES phi 2, LES phi 3 and LES phi 4) that are members of the Siphoviridae. All were induced from LESB58 by norfloxacin. Lytic production of LES phi 2 was considerably higher than that of LES phi 3 and LES phi 4. Each phage was capable of both lytic and lysogenic infection of the susceptible P. aeruginosa host, PAO1, producing phage-specific plaque morphologies. In the PAO1 host background, the LES phi 2 prophage conferred immunity against LES phi 3 infection and reduced susceptibility to LES phi 4 infection. Each prophage was less stable in the PAO1 chromosome with substantially higher rates of spontaneous phage production than when residing in the native LESB58 host. We show that LES phages are capable of horizontal gene transfer by infecting P. aeruginosa strains from different sources and that type IV pili are required for infection by all three phages. Conclusions: Multiple inducible prophages with diverse infection properties have been maintained in the LES genome. Our data suggest that LES phi 2 is more sensitive to induction into the lytic cycle or has a more efficient replicative cycle than the other LES phages.