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Genetic characteristics: Plasmids and prophages have been identified in S. Typhi. For example, a resistant plasmid of 218 kb and seven prophages were detected in S. Typhi CT18. Up Parkhill et al. sequenced the complete genome (4,809,037 bp) of Salmonella Typhi (CT18) and found this isolate exhibited substantial genetic decay with over 200 pseudogenes, including homologs of key Salmonella Typhimurium virulence genes (e.g., sopE2 and fliB), potentially explaining its human-restricted host specificity. This isolate carries two plasmids: a 218,150 bp multidrug-resistant IncHI1 plasmid (pHCM1) and a 106,516 bp cryptic plasmid (pHCM2) phylogenetically linked to a virulence plasmid of Yersinia pestis. Additionally, it harbors up to 15 Salmonella pathogenicity islands (SPIs ) have been detected in S. Typhi, these include SPI-1 to 71–7, 9, 11 to 1311–13, and 15 to 1815–18).

Comparative genomics and phylogenetic analyses have shown that that S. Typhi (i) had a monophyletic origin approx. 50,000 years ago (ii) has suffered reductive evolution (approx. 200 genes are functionally inactive), (iii) belongs to clade A, according to den Bakker et al. (2011) classification, and (iviii) has 349 gene families unique to this serovar.A number of projects have sequenced the genome of strains of S. Typhi; these studies have concluded that S. Typhi is a highly clonal serovar that emerged approx. 30,000-50,000 years ago. In addition, studies have found that horizontal gene transfer plays an important role in the evolution S. Typhi, this included the acquisition of plasmids encoding multiple antibiotic resistant genes in multidrug resistant strains.

Animal reservoir: Salmonella Typhi is a human-restricted serovar.

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