"This training is bound for glory": selection by experimental evolution of a bacteriophage with expanded host-range and increased virulence

Phage host-range expansion is predicted to be at the cost of lower mean fitness. We aimed at following the adaptive walks of a virulent phage (Tequintavirus) evolving in a spatially variable environment composed of four susceptible and four resistant strains (Salmonella enterica enterica pv Tennessee, sequence types ST5018 and ST319 respectively). We evolved a single ancestor through serial passages on the non-coevolving bacterial strains following Appelmans' protocol and obtained several evolved phage populations with expanded host-range and increased virulence. Phage populations sequencing revealed multiple mutations appearing at the same loci (parallel mutations), notably on exo- and endo-nuclease, dUTPase and caudal proteins. Two parallel mutations present on the long tail fiber gene showed to become fixed within the population before the other parallel mutations. Introduction by reverse-genetics of these two mutations into the ancestral genome expanded host-range but not virulence. ### Competing Interest Statement The authors have declared no competing interest.