Complementary killing activities of and phages on planktonic and sessile PAO1 derivatives.

Four phages active against a representative panel of strains were chosen with the goal of using them for phage therapy. Two were belonging to the species, and two to the species. The receptor of the phage Ab27 had already been characterized as the O-antigen chain of the lipopolysaccharides, whereas no information was available at the onset of this work on the receptor used by the phages. We show that this receptor is the surface polysaccharide Psl, an important component of the biofilm matrix. Remarkably, the phages were more active against PAO1 in minimal medium compared to rich medium. This was correlated with larger amounts of Psl bound at the bacterial surface during exponential growth in minimal medium, compared to the rich medium. Phages prevented biofilm growth when applied early after biofilm formation on a medical endotracheal tube, as well as in 96-well plates, and acted more slowly on mature biofilms. No biofilm overgrowth was observed when applying the two phage species combination, over a 48 h period of imaging by confocal microscopy. Genetic mutants resistant to each phage arose at a frequency of 10 to 10 per generation, and most -resistant mutants were sensitized to the phage. The combination of the selected phages has promising properties that are relevant in the framework of phage therapy.