Microbiome sequencing has shown that phage and bacteria coexist and co-evolve in complex communities (Stern et al.,2012, Weitz et al.,2013). Since, microbial communities support all life forms on the earth and bacteriophages are potential antimicrobial agents, it is important to understand the molecular details of phage evolution. A major challenge of phage biology and evolution is to determine the dynamics and driving forces of host range expansion.
A phage can target one specific host (specialist), or many hosts (generalist) (Koskella & Meaden, 2013). Generalists and specialists are both found in natural and clinical microbial communities (de Jonge et al., 2019). However, microbiome studies typically take a “snapshot” of phage diversity at one or a few time points, so it’s difficult to know dynamics of generalism and specialism over time. Experimental evolution is a useful tool to explore the genetic and phenotypic changes in phage and their hosts during evolution in a controlled laboratory environment (Buckling & Brockhurst, 2012). Here we test whether generalism is an intermediate state during the evolution of a specialist by evolving a novel T7-like phage (JB) in presence of diverse strains of Escherichia coli. We found that JB quickly adapted to new host in the presence of original host by diversifying into at least two subpopulations: one that specialized on the original host and new type that could infect the new host. Repeating this experiment across a range of host E. coli strains, we found that the new types were sometimes specialists on the new host, and sometimes generalists. This data suggests that generalism may be a transient state during the evolution of specialists, but that could be stable under certain conditions.