Characterisation of IncI1 plasmids associated with change of phage type in isolates of Salmonella enterica serovar Typhimurium

Background Acquisition of IncI1 plasmids by members of the Enterobacteriaceae sometimes leads to transfer of antimicrobial resistance and colicinogeny as well as change of phage type in Salmonella Typhimurium. Isolates of S. Typhimurium from a 2015 outbreak of food poisoning were found to contain an IncI1 plasmid implicated in change of phage type from PT135a to U307 not previously reported. The origin of the changes of phage type associated with this IncI1 plasmid was investigated. In addition, a comparison of its gene composition with that of IncI1 plasmids found in local isolates of S. Typhimurium typed as U307 from other times was undertaken. This comparison was extended to IncI1 plasmids in isolates of phage types PT6 and PT6 var. 1 which are thought to be associated with acquisition of IncI1 plasmids. Results Analysis of IncI1 plasmids from whole genome sequencing of isolates implicated a gene coding for a 1273 amino acid protein present only in U307 isolates as the likely source of change of phage type. The IncI1 plasmids from PT6 and PT6 var. 1 isolates all had the ibfA gene present in IncI1 plasmid R64. This gene inhibits growth of bacteriophage BF23 and was therefore the possible source of change of phage type. A fuller comparison of the genetic composition of IncI1 plasmids from U307 isolates and PT6 and PT6 var. 1 isolates along with two IncI1 plasmids from S. Typhimurium isolates not showing change of phage type was undertaken. Plasmids were classified as either ‘Delta’ or ‘Col’ IncI1 plasmids according to whether genes between repZ and the rfsF site showed high identity to genes in the same location in R64 or ColIb-P9 plasmids respectively. Comparison of the tra gene sets and the pil gene sets across the range of sequenced plasmids identified Delta and Col plasmids with almost identical sequences for both sets of genes. This indicated a genetic recombination event leading to a switch between Delta and Col gene sets at the rfsF site. Comparisons of other gene sets showing significant variation among the sequenced plasmids are reported. Searches of the NCBI GenBank database using DNA and protein sequences of interest from the sequenced plasmids identified global IncI1 plasmids with extensive regions showing 99 to 100% identity to some of the plasmids sequenced in this study indicating evidence for widespread distribution of these plasmids. Conclusion Two genes possibly associated with change of phage type were identified in IncI1 plasmids. IncI1 plasmids were classified as either ‘Delta’ or ‘Col’ plasmids and other sequences of significant variation among these plasmids were identified. This study offers a new perspective on the understanding of the gene composition of IncI1 plasmids. The sequences of newly sequenced IncI1 plasmids could be compared against the regions of significant sequence variation identified in this study to understand better their overall gene composition and relatedness to other IncI1 plasmids in the databases.