Correlation between the loss of plasmid DNA and the transition from virulent phase I to avirulent phase II in Shigella sonnei

Transition from virulent phase I to non virulent phase II in five strains of Shigella sonnei was studied by immunofluorescence and plasmid electrophoresis. High frequency loss of phase I specific antigens was observed by use of fluorescent phase specific antibodies. Agarose gel electrophoresis of bacterial lysates showed that transition from phase I to phase II is associated with the loss of extrachromosomal DNA.