Castledine, M.; Szczutkowska, Z.; Matthews, A.; Walsh, S. K.; Lewis, R.; Kay, S.; Willment, J. A.; Brown, G. D.; Buckling, A.

Phage therapy, the use of viruses that infect bacteria (bacteriophages), is a promising complement to antibiotics during the antimicrobial resistance crisis, but treatment success is very variable. Evolution of bacterial resistance to phage and phage counter-resistance (coevolution) during therapy may explain some of this variation, the dynamics of which may be affected by interactions with the patients immune system. Here, we examine how a pathogenic bacterium, Pseudomonas aeruginosa coevolves with two clinically relevant bacteriophages (14-1 and PNM) when in the presence of macrophages (RAW 264.7 cell line). We show macrophages reduced the rate by which bacteria were killed by bacteriophages, likely by reducing bacteria-phage contact rates. Over evolutionary time-scales, macrophages increased the proportion of phage resistant bacteria compared to where macrophages were absent. These differences in resistance rates were likely driven by the early advantage in density offered by macrophages to bacteria, exclusion of PNM from the phage cocktail which otherwise increased in frequency in the absence of macrophages. Consequently, macrophages significantly altered the short- and long-term efficacy of a phage cocktail. In line with a growing body of work, our results suggest that the immune system can reduce the efficacy of phage therapy, potentially driving variable outcomes in therapy success in patients.