Castledine, M.; Padfield, D.; Schoeman, M.; Berry, A.; Buckling, A.

Bacteriophages can be important drivers of bacterial densities, and therefore microbial community composition and function. These ecological interactions are likely to be greatly affected by evolutionary dynamics, because bacteria can rapidly evolve resistance to phage while phage can reciprocally evolve to increase infectivity. Most studies to date have explored eco-evolutionary dynamics using isolated pairs of bacteria-phage but in nature, multiple bacteria and phages coexist and (co)evolve simultaneously. How coevolution plays out in this context is poorly understood. Here, we examine how three coexisting soil bacteria (Ochrobactrum sp., Pseudomonas sp., and Variovorax sp.) interact and evolve with three species-specific bacteriophages over eight weeks of experimental evolution, both as host-parasite pairs in isolation and as a mixed community. Across all species phage resistance evolution was inhibited in polyculture, with the most pronounced effect on Ochrobactrum. Between bacteria-phage pairs there were also substantial differences in the effect of phage on host densities, and evolutionary dynamics including whether pairs coevolved. These contrasts emphasise the difficulty in generalising from monoculture to polyculture, and between bacteria-phage pairs to wider systems. Future studies should consider how multiple bacteria and phage pairs interact simultaneously to better understand how coevolutionary dynamics happen in natural communities.