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Genetic and physical characterization of trimethoprim resistance plasmids from Shigella sonnei and Shigella flexneri
1Department of Bacteriology, Hospital for Sick Children, Toronto, Ont., Canada.
Abstract:
Analysis of six Shigella flexneri and four S. sonnei isolates with trimethoprim (Tp) resistance from clinical cases in Ontario has shown that, in all isolates, the Tp resistance is mediated by gene(s) on conjugative, multiple antibiotic-resistance plasmids. The physical and genetic characterization of these plasmids revealed that there are three different Tp resistance plasmids. One group, composed of all six S. flexneri plasmids, consists of plasmids which are about 70 megadaltons (MDa) and inhibit the fertility of an Escherichia coli Hfr strain (Fi+). A representative member of this group, pPT4, demonstrates a weak incompatibility reaction with IncFl plasmid R455-2. Another group, three of the four S. sonnei plasmids, contains plasmids which are about 43 MDa, Fi-, and mediate propagation of phage PRD1. The third group, the remaining S. sonnei plasmid, is 53 MDa, fi+, mediates propagation of phages fd and MS2, and is incompatible with IncFII plasmid R100. These plasmids also have been differentiated by restriction endonuclease fragment profiles. Analysis of pPT4 has revealed that the Tp resistance of this plasmid is transposable. The transposon, Tn536, is different from previously described Tp resistance transposons; it is 16 MDa, and in addition to Tp, it encodes resistance to mercuric chloride ions, spectinomycin, streptomycin, and sulfonamides.
Insights
Trimethoprim resistance in Shigella isolates from Ontario is due to genes on conjugative plasmids. Three distinct trimethoprim resistance plasmids were identified, with one containing a novel transposon, Tn536.
Area of Science:
- Microbiology
- Genetics
- Molecular Biology
Background:
- Trimethoprim resistance is a growing concern in bacterial infections.
- Shigella species are significant human pathogens.
- Understanding the genetic basis of antibiotic resistance is crucial for effective treatment.
Purpose of the Study:
- To characterize trimethoprim resistance plasmids in Shigella flexneri and Shigella sonnei isolates.
- To investigate the genetic elements responsible for trimethoprim resistance.
- To differentiate between various resistance plasmids found in clinical isolates.
Main Methods:
- Plasmid isolation and characterization (size, fertility inhibition, phage propagation).
- Restriction endonuclease fragment profiling.
- Transposon analysis of trimethoprim resistance genes.
Main Results:
- Three distinct trimethoprim resistance plasmids were identified in Shigella isolates.
- Plasmids varied in size (43-70 MDa), fertility inhibition, and phage propagation.
- A novel transposon, Tn536, conferring trimethoprim resistance and other antibiotic resistances, was identified on one plasmid.
Conclusions:
- Conjugative plasmids are the primary mediators of trimethoprim resistance in these Shigella isolates.
- The identified plasmids represent distinct genetic entities with varying characteristics.
- Tn536 is a novel transposon contributing to multidrug resistance in Shigella.