Emergence of a Globally Dominant IncHI1 Plasmid Type Associated with Multiple Drug Resistant Typhoid

Typhoid fever, caused by Salmonella enterica serovar Typhi ( S . Typhi), remains a serious global health concern. Since their emergence in the mid-1970s multi-drug resistant (MDR) S . Typhi now dominate drug sensitive equivalents in many regions. MDR in S . Typhi is almost exclusively conferred by self-transmissible IncHI1 plasmids carrying a suite of antimicrobial resistance genes. We identified over 300 single nucleotide polymorphisms (SNPs) within conserved regions of the IncHI1 plasmid, and genotyped both plasmid and chromosomal SNPs in over 450 S . Typhi dating back to 1958. Prior to 1995, a variety of IncHI1 plasmid types were detected in distinct S . Typhi haplotypes. Highly similar plasmids were detected in co-circulating S . Typhi haplotypes, indicative of plasmid transfer. In contrast, from 1995 onwards, 98% of MDR S . Typhi were plasmid sequence type 6 (PST6) and S . Typhi haplotype H58, indicating recent global spread of a dominant MDR clone. To investigate whether PST6 conferred a selective advantage compared to other IncHI1 plasmids, we used a phenotyping array to compare the impact of IncHI1 PST6 and PST1 plasmids in a common S . Typhi host. The PST6 plasmid conferred the ability to grow in high salt medium (4.7% NaCl), which we demonstrate is due to the presence in PST6 of the Tn 6062 transposon encoding BetU.