Selecting for lysis in a temperate phage population

Temperate phages possess the ability to integrate their genetic material into the host genome, a feature that diminishes their therapeutic value. A team of researchers sought to determine if repeated selection for lytic replication could transform a temperate phage into an obligate lytic form. To achieve this, they propagated a mixture of three temperate coliphages through a modified Appelmans protocol, wherein the phage populations were independently passed through eight Escherichia coli hosts.

Whole-genome sequencing of the resulting populations unveiled significant alterations in composition and mutation frequencies.

Initially, Uetakevirus phage 1362 was undetectable after the first round of propagation. In contrast, closely related P2-like phages within the cocktail quickly became dominant in the evolving populations. Mutations continuously accumulated in lysis- and lysogeny-associated genes over successive rounds of selection. The researchers conducted a longitudinal genomic analysis, which revealed a rapid fixation of mutations within the integrase coding region.

Additionally, variants in the lysogeny-related genes lysA, lysB, and holin increased in frequency across the successive rounds of selection.

Protein structure modeling indicated that the selected mutations were localized to regions with potential functional significance. These regions included the DNA-binding domain of integrase and a surface-exposed region of LysA. By demonstrating that sustained selection for lytic replication could drive rapid and repeatable genomic adaptation in temperate phage populations, this study highlights the potential of phage therapy through directed evolution.