Clara Torres-Barcelo

1 Introduction Understanding the breadth and evolutionary constraints of phage host range is crucial for predicting microbial community dynamics and developing effective phage-based therapies. A central question in phage ecology concerns whether specialisation imposes fitness trade-offs, as a consequence of the difficulty to optimise infection on very diverse hosts (Visher and Boots 2020).

Abstract Phages are typically known for having a limited host range, targeting various strains within a specific bacterial species. However, factors like the phylogeny or epidemiology of host bacteria are often disregarded, despite their potential influence on phage specialization and virulence. This research utilizes a new "phylogenetic host range index" that accounts for the genetic diversity of bacterial hosts, to classify phages into specialists and generalists accurately.