Resistance evolution against phage combinations depends on the timing and order of exposure

Phage therapy is a promising alternative to chemotherapeutic antibiotics for the treatment of bacterial infections. However, despite recent clinical uses of combinations of phages to treat multidrug resistant infections, a mechanistic understanding of how bacteria evolve resistance against multiple phages is lacking, limiting our ability to deploy phage combinations optimally. Here we show, using Pseudomonas aeruginosa and pairs of phages targeting shared or distinct surface receptors, that the timing and order of phage exposure determines the strength, cost and mutational basis of resistance. Whereas sequential exposure allowed bacteria to acquire multiple resistance mutations effective against both phages, this evolutionary trajectory was prevented by simultaneous exposure, resulting in quantitatively weaker resistance. The order of phage exposure determined the fitness costs of sequential resistance, such that certain sequential orders imposed much higher fitness costs than the same phage pair in the reverse order. Together these data suggest that phage combinations can be optimised to limit the strength of evolved resistances whilst maximising their associated fitness costs to promote the long-term efficacy of phage therapy.