[Formation of F'-episomes in Flexner shigellae].

Genetic characteristics of Shigella flexneri virulent donor strain 3s, isolated in the laboratory, are studied. The data obtained showed that Sh. flexneri 3s 6832 is of F'protype it did not transmit even closely located chromosome markers, except pro+. The transition of pro+ marker was reproduced in recA- strain. F'pro plasmid was eliminated with a frequency similar to that of F'lac factor in the control strain of Escherichia coli. The study of virulence by means of keratoconjunktival test revealed its retaining in clones which lost F'pro plasmid. Probably, KCP-A gene retained in the chromosome of F'pro strain. Crossing with proA- and procC- C. coli strains revealed a small length of F'pro episome, enly ProA+ hybrids being obtained. A complex mechanism of F'system formation in interspecies genetic recombination between E. coli K-12 and Sh. flexneri 3s is demonstrated.