Introduction. Salmonella enterica serovar Typhimurium is a food-borne pathogen and a leading cause of gastroenteritis in humans. Recently, we sequenced a phage type DT108 strain (L945) and found reads with high similarity to both S. Typhi strain CT18 plasmid pHCM2 and bacteriophage SSU5. In this study, we report the full sequence of pSTM_Φ, its evolutionary relationships to other phage-like plasmids and a S. Typhimurium phage, and its prevalence in S. Typhimurium.

Results. The presence of this phage-like plasmid was examined in a collection of 284 S. Typhimurium isolates using PCR of the parB gene and only one other isolate (L946) was found to carry the phage-like plasmid suggesting that it is infrequently present amongst S. Typhimurium isolates. pSTM_Φ is a circular phage-like plasmid of 107.7 kb encoding 132 coding regions (ORFs) with the majority of the ORFs encoding hypothetical proteins. pSTM_Φ does not contain any known virulence or resistance genes suggesting its cryptic nature. Further studies are necessary to gain an insight into the overall biological effect of this plasmid. Comparative analysis with other closely related phage-like plasmids and the SSU5 phage revealed that there were four divergent lineages of phage-like plasmids found in the family of Enterobacteriaceae. Although the S. Typhimurium bacteriophage SSU5 shared considerable homology with pSTM_Φ, clearly the two have separate origins, as the latter is more related to the S. Typhi plasmid pHCM2 and shared most recent common ancestor with Yersinia pestis plasmid pMT1.

Conclusion. Phage-like plasmids are an emerging family of mobile genetic elements of bacteria and may play an important role in adaptation.