Salmonella Typhimurium-specific bacteriophage ΦSH19 and the origins of species specificity in the Vi01-like phage family

Background: Whole genome sequencing of bacteriophages suitable for biocontrol of pathogens in food products is a pre-requisite to any phage-based intervention procedure. Trials involving the biosanitization of Salmonella Typhimurium in the pig production environment identified one such candidate, Phi SH19. Results: This phage was sequenced and analysis of its 157,785 bp circular dsDNA genome revealed a number of interesting features. Phi SH19 constitutes another member of the recently-proposed Myoviridae Vi01-like family of phages, containing S. Typhi-specific Vi01 and Shigella-specific SboM-AG3. At the nucleotide level Phi SH19 is highly similar to phage Vi01 (80-98% pairwise identity over the length of the genome), with the major differences lying in the region associated with host-range determination. Analyses of the proteins encoded within this region by Phi SH19 revealed a cluster of three putative tail spikes. Of the three tail spikes, two have protein domains associated with the pectate lyase family of proteins (Tsp2) and P22 tail spike family (Tsp3) with the prospect that these enable Salmonella O antigen degradation. Tail spike proteins of Vi01 and SboM-AG3 are predicted to contain conserved right-handed parallel beta-helical structures but the internal protein domains are varied allowing different host specificities. Conclusions: The addition or exchange of tail spike protein modules is a major contributor to host range determination in the Vi01-like phage family.