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Salmonella plasmids of the pre-antibiotic era
Abstract:
In order to provide a profile of the plasmid gene pool in Salmonella prior to the clinical use of antibiotics, a molecular genetic analysis was made of plasmids in strains collected by E.D.G. Murray between 1917 and 1954. These pre-antibiotic era (PAE) salmonellae contain conjugative plasmids of the same incompatibility groups as contemporary enterobacterial plasmids. Upon analysis of total plasmid content, 42 plasmids, sized between 23 and 72 MDa, were found. We defined and investigated six groups of these PAE Salmonella plasmids in terms of three groups of genes; those involved in plasmid maintenance and incompatibility, DNA repair and virulence. Of the five groups, three were replicon-typed to groups IncI1, IncX and IncFII; one group exhibited no homology to contemporary Inc/Rep probes, and one group represented virulence plasmids containing a common plasmid-partitioning locus. The results indicated that most of the PAE groups were progenitors of contemporary R-plasmids, except for the virulence plasmids, which have generally not evolved as vectors of antibiotic resistance.
Insights
This study analyzed pre-antibiotic era Salmonella plasmids, revealing they share incompatibility groups with modern plasmids. Most ancient plasmid groups are ancestors of contemporary antibiotic resistance plasmids.
Area of Science:
- Microbiology
- Genetics
- Evolutionary Biology
Background:
- Antibiotic resistance is a growing public health concern.
- Understanding the historical evolution of plasmids is crucial for combating resistance.
- Salmonella strains from the pre-antibiotic era offer insights into early plasmid dynamics.
Purpose of the Study:
- To characterize the plasmid gene pool in Salmonella before widespread antibiotic use.
- To investigate the genetic makeup and evolutionary relationships of pre-antibiotic era (PAE) plasmids.
- To compare PAE plasmids with contemporary enterobacterial plasmids.
Main Methods:
- Molecular genetic analysis of plasmids from Salmonella strains collected between 1917 and 1954.
- Plasmid content analysis to identify and size plasmids.
- Gene group investigation (maintenance, incompatibility, DNA repair, virulence) and replicon typing.
Main Results:
- Identified 42 conjugative plasmids (23-72 MDa) in PAE Salmonella.
- Classified PAE plasmids into six groups based on gene content.
- Three groups matched contemporary incompatibility groups (IncI1, IncX, IncFII); one showed no homology; one contained virulence genes.
- Most PAE plasmid groups are likely progenitors of modern R-plasmids.
Conclusions:
- PAE Salmonella harbored diverse plasmids with genetic elements similar to contemporary strains.
- Most PAE plasmid groups represent ancestral forms of modern antibiotic resistance plasmids.
- Virulence plasmids from this era did not typically evolve as vectors for antibiotic resistance.