Phage infection reinstates antibiotic sensitivity in MDR Pseudomonas aeruginosa

The emergence of antibiotic resistance among bacterial pathogens is a significant public health threat affecting humans worldwide. In Europe, Pseudomonas aeruginosa contributes to almost 9% of overall multi-drug-resistant (MDR) infections. Alternative methods for controlling MDR pathogens have been explored for several decades. Bacteriophage therapy is one of the oldest and most efficient alternative solutions. The study described in this thesis began with the isolation and characterization of 25 MDR P. aeruginosa clinical strains and eigth novel lytic phages. The investigation disclosed the infection with two phage isolates, PIAS and PAPSZ1, led to the sequential appearance of phage-resistant colonies with two phenotypes (green and brown). We examined the evolutionary basis for the two types of mutants and uncovered phage mutants capable of infecting green mutants. Simultaneously we also learned that PIAS phage infected the host via the OrpM-MexXY system involved in drug efflux. Thus, the PIAS-resistant mutants decreased the minimum inhibitory concentrations (MIC) for several non-effective antibiotics. After this new insight into the evolutionary arms race between hosts and phages, we decided to use this window to comprehensively eradicate mutants by treating MDR strain with previously resistant antibiotics combined with PIAS phage. The in vitro study with PIAS phage-antibiotic combination completely prevented the formation and growth of mutants. We tested the same strategy in an in vivo rescue experiment in the mouse lung infection model, when combined with PIAS phage and fosfomycin. The combination therapy saved 75% of the animals. Later, we used PAPSZ1 phage to investigate whether phage mutants can suppress bacterial resistance. We isolated multiple PAPSZ1 mutants after a continuous infection cycle, which can block or suppress bacterial resistance and mutant formation and broaden the host specificity of the phages. Phages like PIAS and PAPSZ1 offer a unique window that can exploit to eradicate MDR bacteria. This study highlights the importance of preliminary and detailed examinations of phage-host bacterium interactions preceding the application of a given phage. The experimental data in this thesis shows that studying phage-host bacterium interactions and coevolution will help to utilize phage therapy’s full potential when treating MDR infections.