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The virulence plasmid of Salmonella typhimurium is self-transmissible
1Department of Molecular Microbiology and Immunology, Oregon Health Sciences University, Portland, Oregon 97201-3098, USA. ahmerb@ohsu.edu
Abstract:
Most isolates of Salmonella enterica serovar Typhimurium contain a 90-kb virulence plasmid. This plasmid is reported to be mobilizable but nonconjugative. However, we have determined that the virulence plasmid of strains LT2, 14028, and SR-11 is indeed self-transmissible. The plasmid of strain SL1344 is not. Optimal conjugation frequency requires filter matings on M9 minimal glucose plates with a recipient strain lacking the virulence plasmid. These conditions result in a frequency of 2.9 x 10(-4) transconjugants/donor. Matings on Luria-Bertani plates, liquid matings, or matings with a recipient strain carrying the virulence plasmid reduce the efficiency by up to 400-fold. Homologs of the F plasmid conjugation genes are physically located on the virulence plasmid and are required for the conjugative phenotype.
Insights
The Salmonella Typhimurium virulence plasmid is self-transmissible, contrary to previous reports. Optimal conditions for this plasmid transfer were identified, involving specific mating methods and recipient strains.
Area of Science:
- Microbiology
- Bacterial Genetics
Background:
- Most Salmonella enterica serovar Typhimurium isolates possess a 90-kb virulence plasmid.
- This plasmid was previously characterized as mobilizable but nonconjugative.
Purpose of the Study:
- To investigate the conjugative ability of the Salmonella Typhimurium virulence plasmid.
- To identify optimal conditions for the plasmid's self-transmission.
Main Methods:
- Conjugation experiments were performed using different Salmonella Typhimurium strains (LT2, 14028, SR-11, SL1344).
- Filter matings on M9 minimal glucose plates and liquid matings were compared.
- Recipient strains with and without the virulence plasmid were utilized.
- Homology analysis of plasmid genes with known conjugation factors was conducted.
Main Results:
- The virulence plasmids of Salmonella Typhimurium strains LT2, 14028, and SR-11 were found to be self-transmissible.
- The virulence plasmid of strain SL1344 was determined to be non-self-transmissible.
- Optimal conjugation frequency (2.9 x 10(-4) transconjugants/donor) was achieved using filter matings on M9 minimal glucose plates with a plasmid-free recipient.
- Less efficient transfer occurred on Luria-Bertani plates, in liquid matings, or with recipient strains already harboring the virulence plasmid, with efficiency reduced up to 400-fold.
- Physical location and requirement of F plasmid conjugation gene homologs on the virulence plasmid for self-transmission were confirmed.
Conclusions:
- The Salmonella Typhimurium virulence plasmid is capable of self-conjugation, challenging prior assumptions.
- Specific environmental and genetic conditions significantly influence the efficiency of virulence plasmid transfer.
- The presence of F plasmid conjugation gene homologs is crucial for the observed conjugative phenotype of the virulence plasmid.