Characterisation and distribution of a cryptic Salmonella typhi plasmid pHCM2

pHCM2 is a 106 kbp cryptic plasmid harboured by Salmonella typhi CT18, originally isolated from a typhoid patient in Vietnam. The genome of S. typhi CT18, including pHCM2, has recently been completely sequenced and annotated. Bioinformatic analysis revealed that 57% of the coding sequences (CDSs) encoded on pHCM2 display over 97% DNA sequence identity to the virulence-associated plasmid of Yersinia pestis, pFra. pHCM2 encodes no obvious virulence-associated determinants or antibiotic resistance genes but does encode a wide array of putative genes directly related to DNA metabolism and replication. PCR analysis of a series of S. typhi isolates from Vietnam detected pHCM2-related DNA sequences in some S. typhi isolated before, but not after, 1994. Similar pHCM2-related sequences were also detected in S. typhi isolated from other regions of South East Asia and Pakistan but not elsewhere in the world.