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Involvement of a plasmid in the invasive ability of Shigella flexneri
Abstract:
Representative Shigella flexneri strains were studied to determine whether plasmids are involved in their virulence. All invasive S. flexneri strains, irrespective of serotype, were found to harbor a large plasmid of approximately 140 megadaltons in size, although some strains carried additional plasmid species. Spontaneous variants of strains of serotypes 1, 2, and 5 had lost this 140-megadalton plasmid and had concomitantly become avirulent, i.e., could neither invade HeLa cell monolayers nor produced keratoconjunctivitis in guinea pigs. To monitor plasmid transfer, the 140-megadalton plasmid of strain M90T (serotype 5) was tagged with the kanamycin resistance transposon Tn5. This tagged plasmid, pWR110, was not self-transmissible, but was mobilized by one of several different conjugative plasmids into avirulent derivatives of the heterologous serotypes 1 and 2 which had lost the comparable large plasmid. Transconjugants of both serotypes which had received pWR110 regained virulence. These data directly demonstrate that this large S. flexneri plasmid encodes or regulates some function(s) required for epithelial cell penetration.
Insights
Large plasmids in Shigella flexneri are essential for virulence. Loss of this plasmid renders strains avirulent, unable to invade cells or cause disease. Transferring the plasmid restores virulence, confirming its critical role.
Area of Science:
- Microbiology
- Molecular Biology
- Infectious Diseases
Background:
- Shigella flexneri is a bacterial pathogen causing bacillary dysentery.
- The role of plasmids in S. flexneri virulence has been investigated.
- Invasive S. flexneri strains typically possess a large plasmid.
Purpose of the Study:
- To determine if plasmids are involved in Shigella flexneri virulence.
- To identify the specific plasmid responsible for virulence factors.
- To demonstrate the transferability and functional role of the virulence plasmid.
Main Methods:
- Analysis of plasmid content in invasive S. flexneri strains.
- Creation of spontaneous avirulent variants lacking the large plasmid.
- Plasmid tagging with a transposon (Tn5) for tracking.
- Mobilization of the tagged plasmid into avirulent strains via conjugation.
- Assessment of virulence in transconjugant strains (HeLa cell invasion, keratoconjunctivitis model).
Main Results:
- All invasive S. flexneri strains harbored a ~140-megadalton plasmid.
- Loss of this plasmid resulted in avirulence (inability to invade HeLa cells or cause keratoconjunctivitis).
- Transfer of the tagged 140-megadalton plasmid (pWR110) restored virulence in previously avirulent strains.
Conclusions:
- The large ~140-megadalton plasmid of S. flexneri is essential for virulence.
- This plasmid encodes or regulates functions necessary for epithelial cell penetration.
- Plasmid transfer is a mechanism for restoring virulence in S. flexneri.