Genetic and physical evidence for plasmid control of Shigella sonnei form I cell surface antigen

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Abstract
Virulent S. sonnei synthesize a surface antigen [Ag] (form I) which may be 1 of several requirements needed for this host to invade [guinea pig] epithelial cells. Upon restreaking on agar media, form I cells readily and irreversibly generate form II cells that lack the form I antigen. All form II cells are avirulent. Plasmid DNA of form I and II cells of 4 different S. sonnei isolates, obtained from different areas of the world, was analyzed by agarose gel electrophoresis. A large plasmid (.apprx. 120 megadaltons in 3 of the strains) present in form I cells was always absent from form II derivatives. Attempts to transfer conjugally only this large plasmid from form I to genetically marked form II cells were unsuccessful. A composite molecule, apparently formed by recombination between the large form I plasmid and a self-transmissible plasmid, tranferred the form I trait. Transconjugant S. sonnei strains acquiring the form I Ag could retransfer this trait to S. sonnei, S. flexneri or Salmonella typhi. S. sonnei form I Ag synthesis is mediated by a large plasmid which is lost spontaneously at a relatively high frequency from S. sonnei strains.