The biology of IncI2 plasmids shown by whole-plasmid multi-locus sequence typing
1U.S. National Poultry Research Center, USDA Agricultural Research Service, 950 College Station Road, Athens, GA 30605, USA.
Abstract:
IncI2 type plasmids are medium-sized (~55-80 kb) conjugative plasmids that have been found carrying important antimicrobial resistance genes but have also been frequently found as cryptic plasmids. The DNA sequences for 147 fully sequenced IncI2 plasmids were studied by a whole-plasmid multi-locus sequence typing (wpMLST) scheme. A total of 171 loci were identified of which 52 were considered core (carried by greater than 95% of the plasmids). Most of the plasmids carrying the antimicrobial gene mcr-1 were in a distinct clade while most of the antimicrobial gene free plasmids were more distantly related. However, the host strains of bacteria were disparate for both groups of plasmids, showing that conjugal transfer of IncI2 plasmid is frequent. The mcr-1 gene was likely to have been introduced into IncI2 plasmids multiple times. It was also observed that the genes for conjugation showed significant linkage disequilibrium despite substantial diversity for most of those genes. Genes associated with biofilm formation were also among the core genes. The core genes can be considered the cohesive unit that defines the IncI2 plasmid group. Given the role conjugation can play in biofilm formation, it was concluded that conjugation is an active survival strategy for IncI2 plasmids. The IncI2 plasmid will have selective advantage when the plasmid-bearing bacteria are introduced to a new animal host that carries potential conjugal mates.
Insights
IncI2 plasmids are key mobile genetic elements carrying antimicrobial resistance genes. Whole-plasmid multi-locus sequence typing reveals core genes essential for IncI2 plasmid identity and conjugation, highlighting their role in bacterial survival.
Area of Science:
- Microbiology
- Genetics
- Molecular Biology
Background:
- IncI2 plasmids are medium-sized conjugative plasmids often carrying antimicrobial resistance genes.
- These plasmids are frequently identified as cryptic, lacking known functional genes.
- Understanding the IncI2 plasmid population structure is crucial for tracking antimicrobial resistance.
Purpose of the Study:
- To characterize the IncI2 plasmid population using a whole-plasmid multi-locus sequence typing (wpMLST) scheme.
- To identify core genes defining the IncI2 plasmid group.
- To investigate the evolutionary history and dissemination of antimicrobial resistance genes, such as mcr-1, within IncI2 plasmids.
Main Methods:
- Whole-plasmid multi-locus sequence typing (wpMLST) was applied to 147 fully sequenced IncI2 plasmids.
- Analysis of 171 identified loci to determine core genes present in >95% of plasmids.
- Phylogenetic analysis to assess the relationship between plasmids carrying mcr-1 and those without.
Main Results:
- 171 loci were identified, with 52 designated as core genes defining the IncI2 plasmid group.
- Plasmids carrying the mcr-1 gene formed a distinct clade, separate from mcr-1-free plasmids.
- Frequent conjugal transfer was inferred from disparate bacterial host strains across plasmid groups.
- Genes for conjugation and biofilm formation were identified as core components.
- The mcr-1 gene has likely been acquired multiple times by IncI2 plasmids.
Conclusions:
- The identified core genes represent the cohesive unit defining the IncI2 plasmid group.
- Conjugation is an active survival strategy for IncI2 plasmids, particularly in new animal host environments.
- IncI2 plasmids gain a selective advantage through efficient conjugal transfer and potential for antimicrobial resistance acquisition.
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