[Effect of the transfer of F1 plasmids of various lengths on Shigella sonnei virulence].

The influence of F' plasmids of different length, carrying the genetically characterized Escherichia coli K-12 chromosomal regions, on the virulence of S. sonnei has been studied. The study has revealed that in S. sonnei (in contrast to S. flexneri) the chromosome carries a recessive gene located in the area of the lactose operon; this gene is essential for the capacity of bacteria for causing keratoconjunctivitis, which has been proved by the transfer of F' plasmids of different length. The above gene is functionally independent of the influence of the F factor as such, as the transconjugants which receive the so-called "intermediate" plasmids carrying the E. coli K-12 chromosomal genes from lac I to tsx retain their virulence. The above gene (or genes) on the chromosome of S. sonnei has been localized to the left of the gene lac I.