Characterization of Siphoviridae Bacteriophage Induction of Lytic Cycle of Prophage in Serratia marcescens

Isolation and characterization of a Salmonella Siphoviriade-like bacteriophage (PNW_Siphoviridae) was shown to lyse wild-type Serratia marcescens in a temperature and strain-dependent manner. Glycosylated residues of lipopolysaccharide (LPS) on WT S. marcescens were hypothesized to be the primary receptor for the phage through isolation of phage resistant mutants, suggesting a mechanism for strain and temperature specificity through LPS variability. To test this, we aimed to adapt PNW_Siphoviridae to infect S. marcescens strain Db11, a non-permissive strain, through a series of broth culture propagations. Lytic plaques were observed and the isolated phage was shown to be a generalist phage, infecting several strains of S. marcescens via plaque assay in the sixth passage (P6). Genomic sequencing and bioinformatic analysis surprisingly revealed that P6 is not an evolved form of PNW_Siphoviridae but rather Salmon_118970_sal3 (PNW_sal3), a prophage from S. marcescens. In this study, we aimed to characterize PNW_sal3 and determine pathways that triggered the expression of the prophage following infection of PNW_Siphoviridae. Bioinformatic and genomic analysis of Salmon_118970_sal3 revealed that this temperate phage can be found in several strains of S. marcescens, with differences in genome composition and infection kinetics that demonstrate the diversity of phages and complexities of host-phage interactions. Due to the sensitivity of WT to PNW_Siphoviridae, we hypothesized that PNW_Siphoviridae infection induced the SOS response in WT S. marcescens, leading the lysogenic prophage to become lytic. Although transcriptomic analysis during infection did not detect upregulation of key SOS genes, we report a general host response of WT S. marcescens to phage infection. Few reports have identified virulent phage as a regulator of prophage induction, and therefore this work could help aid our understanding of phage population dynamics and phage diversity in a ubiquitous bacterium such as S. marcescens.