Available online 18 August 2021, 151534

DT160 and DT56v shared an estimated date of common ancestor between 1769 and 1821.

DT160 replicated at a faster rate than DT56v in vitro.

Neither DT160 nor DT56v were lysed by phage released by the other strain.

A linear plasmid was detected in one DT56v that represents the first report of a Salmonella pBSSB1-family plasmid isolated from S. Typhimurium.

DT160 contained the pSLT virulence plasmid, and the sseJ and sseK2 genes that possibly contributed to its higher reported prevalence.

Salmonella enterica serovar Typhimurium DT160 was the predominant cause of notified human salmonellosis cases in New Zealand from 2000-2010, before it was superseded by another S. Typhimurium strain, DT56 variant (DT56v). Whole genome sequencing and phenotypic testing were used to compare 109 DT160 isolates with eight DT56v isolates from New Zealand animal and human sources. Phylogenetic analysis provided evidence that DT160 and DT56v strains were distantly related with an estimated date of common ancestor between 1769 and 1821. The strains replicated at different rates but had similar antimicrobial susceptibility profiles. Both strains were resistant to the phage expressed from the chromosome of the other strain, which may have contributed to the emergence of DT56v. DT160 contained the pSLT virulence plasmid, and the sseJ and sseK2 genes that may have contributed to the higher reported prevalence compared to DT56v. A linear pBSSB1-family plasmid was also found in one of the DT56v isolates, but there was no evidence that this plasmid affected bacterial replication or antimicrobial susceptibility. One of the DT56v isolates was also sequenced using long-read technology and found to contain an uncommon chromosome arrangement for a Typhimurium isolate. This study demonstrates how comparative genomics and phenotypic testing can help identify strain-specific elements and factors that may have influenced the emergence and supersession of bacterial strains of public health importance.

Genomics

Salmonella

Phage

Epidemic

Plasmid

© 2021 Published by Elsevier GmbH.