Molecular analysis of incHI1 antimicrobial resistance plasmids from Salmonella serovar Typhi strains associated with typhoid fever
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Published on: August 11, 2014
John Wain1, L T Diem Nga, Claire Kidgell
1Wellcome Trust Clinical Research Unit, Department of Biological Sciences, Imperial College, London. j.wain@ic.ac.uk
Abstract:
The first outbreak of multidrug-resistant (MDR) typhoid fever in Vietnam was in 1993, and by 1995 nearly 90% of cases were MDR. Plasmid HCM1, sequenced in full, is an incHI1 plasmid from Salmonella enterica serovar Typhi strain CT18, isolated in Vietnam in 1993. Restriction analysis shows that pHCM1 shares a restriction fragment length polymorphism (RFLP) pattern with plasmids isolated from the first outbreak and 10 of 17 MDR plasmids isolated from sporadic cases occurring at the same time in Vietnam. A core region of pHCM1 has significant DNA sequence similarity to plasmid R27, isolated in 1961 from S. enterica in the United Kingdom. There are five regions of DNA in pHCM1 which are not present in R27. Two of these are putative acquisition regions; the largest is 34.955 kbp in length and includes sequences of several antibiotic resistance genes and several insertion sequences. The borders of this region are defined by two identical IS10 left elements, associated with an inversion of DNA or with a truncated Tn10 element. The second, smaller region is 14.751 kbp and carries a trimethoprim resistance gene dfr14A cassette associated with a class 1 integrase. In 1993 to 1994, restriction analysis revealed some variations in the structures of Salmonella serovar Typhi MDR plasmids which were mapped to the two putative acquisition regions and three smaller variable regions. In 1996 a single RFLP type, RFLP7, was found to carry the dfrA7 and sul-1 genes, which were not present on R27 or pHCM1. This plasmid type appears to have a selective advantage over other plasmids with the same resistance phenotype.
Insights
Multidrug-resistant typhoid fever in Vietnam is linked to plasmid evolution. Plasmid HCM1 shares similarities with older plasmids but contains new acquisition regions with antibiotic resistance genes, driving resistance.
Area of Science:
- Microbiology
- Genetics
- Epidemiology
Background:
- The first outbreak of multidrug-resistant (MDR) typhoid fever in Vietnam occurred in 1993, with nearly 90% of cases becoming MDR by 1995.
- Salmonella enterica serovar Typhi is the causative agent of typhoid fever.
- IncHI1 plasmids are known to carry antibiotic resistance genes and play a role in bacterial pathogen evolution.
Purpose of the Study:
- To investigate the genetic makeup and evolution of multidrug-resistant plasmids in Salmonella Typhi strains from Vietnam.
- To compare the structure of a newly emerged MDR plasmid (pHCM1) with historical plasmids to identify regions associated with resistance acquisition.
- To understand the genetic basis for the increasing prevalence of MDR typhoid fever in Vietnam.
Main Methods:
- Full sequencing of Plasmid HCM1, an incHI1 plasmid from Salmonella Typhi strain CT18.
- Restriction fragment length polymorphism (RFLP) analysis of pHCM1 and other MDR plasmids isolated in Vietnam.
- DNA sequence comparison between pHCM1 and the related plasmid R27 to identify unique regions.
Main Results:
- Plasmid pHCM1 shares RFLP patterns with plasmids from the initial Vietnamese outbreak and other MDR strains.
- pHCM1 contains five DNA regions not found in the older plasmid R27, including two putative acquisition regions.
- The largest acquisition region (34.955 kbp) harbors antibiotic resistance genes and insertion sequences, bordered by IS10 elements.
- A second region (14.751 kbp) contains a trimethoprim resistance gene cassette and a class 1 integrase.
- Variations in MDR plasmid structures were observed between 1993-1994, localized to these acquisition and variable regions.
- A specific RFLP type (RFLP7) carrying dfrA7 and sul-1 genes emerged by 1996, showing a selective advantage.
Conclusions:
- Plasmid evolution, particularly through the acquisition of new genetic elements encoding antibiotic resistance, is a key driver of MDR typhoid fever.
- The identified acquisition regions in pHCM1 are critical sites for the incorporation of resistance genes, contributing to the rapid spread of MDR strains.
- The emergence of RFLP7 suggests ongoing plasmid adaptation and selection for specific resistance profiles in Salmonella Typhi populations.
