FORMULATING PHAGE COCKTAILS AS ADJUVANT THERAPY IN CARBAPENEM-RESISTANT PSEUDOMONAS AERUGINOSA

Phage-antibiotic synergy (PAS) results in bacterial killing beyond the efficacy of either agent alone. Although the main candidates for therapeutic PAS are lytic phages, temperate phages have been re-evaluated in light of PAS. This study explored the potential use of combined temperate phage and ceftolozane/tazobactam (CT) against a carbapenem-resistant Pseudomonas aeruginosa (CRPA). Lytic and temperate phages were isolated and characterized in terms of host range, functional genomics and morphology. Afterwards, 48h biofilms were grown on endotracheal tubes using a CRPA. Biofilms were treated with 2 different phage cocktails (10 8 PFU/mL of four phages), either alone or combined with 5 µg/mL of CT. One cocktail (CK6) consisted exclusively of temperate phages, whereas the other (CK8) included a lytic phage in addition to temperate ones. Repeated treatment doses were administered every 12h. Bacterial counts (log 10 CFU/mL) were assessed every 24h up to 192h. In parallel, the minimal inhibitory concentration of CT was evaluated throughout different treatment groups and timepoints. Phage cocktails and CT alone had only a slight effect on biofilm viability over the treatment period. The combined treatment using both cocktails reduced biofilm viability by 3 logs up to 120h. Emergence of resistance to CT was delayed from 96h with CT alone to 120h with the combined treatment with the CK8. CK6 had no impact on the emergence of resistance to CT. The combination of temperate and lytic phages and CT showed synergistic effects reducing CRPA-biofilm; however, the presence of a lytic phage delayed the emergence of resistance to CT compared to the cocktail that combined only temperate phages. Genomic map overview of a temperate and lytic phage used in the study. (C-D) Biofilm viability (CFU/mL) according to the different groups and timepoints. (E-F) Minimal inhibitory concentrations of CT assessed throughout different treatment groups and timepoints.