Evolutionary strategies of Klebsiella phages in a host-diverse environment

Phage-host interactions have been studied using one or a few phages and hosts in simple, controlled systems. Here, we implemented an experimental design to evolve a 12-phage cocktail in a highly diverse environment, including a combination of 39 Klebsiella spp. capsular types. Our results showed how phages modulate their host range through different strategies, adopting a more generalist lifestyle or maintaining their host specificity, depending on the versatility of receptor binding proteins. Some of these proteins were mutational hotspots during the evolution experiment, mainly due to their intrinsic versatility, allowing a broader or narrower host range, and host diversity in the environment. Other mutations found in evolved phages were associated with evading bacterial defense systems or improving fitness on current hosts. Ecological dynamics between phages and hosts, including prophage activation and recombination events, also determined the fate of phage populations. In addition, single phage evolution experiments were performed to validate the results, allowing a model of receptor binding protein evolution and host range modulation to be proposed. This work is a step forward in the understanding of phage-host interactions and the appropriate implementation of phages as biomedical tools, providing insights into their impact in complex environments such as the human microbiota.