Virulence of Escherichia-shigella Genetic Hybrids for the Guinea Pig.

Falkow, Stanley (Walter Reed Army Institute of Research, Washington, D.C.), H. Schneider, L. S. Baron, and S. B. Formal. Virulence of Escherichia-Shigella hybrids for the guinea pig. J. Bacteriol. 86:1251-1258. 1963.-Genetic recombination studies between donor Escherichia coli and recipient Shigella flexneri 2a strains were employed to examine alterations in the virulence of Shigella hybrids for guinea pigs. The genetic studies indicated that several chromosomal regions of E. coli and S. flexneri are grossly homologous. The frequency of recombination between E. coli and Shigella was decreased, however, in comparison with E. coli x E. coli matings. Moreover, the predominant Shigella hybrid classes acquired only the selected genetic marker, and extensive transfer of the Escherichia genome was detected only occasionally. The virulence studies made use of hybrids with well-defined single markers as well as those with overlapping chromosomal regions. Analysis of over one-half of the chromosome revealed only one chromosomal region, located between the rha(+) and xyl(+) genes, which was essential for virulence. However, hybrids which had received the E. coli pili antigen and fuc(+)-nic(+) determinants exhibited an intermediate virulence. Hybrids carrying a full complement of Shigella genes and the rha(+)-xyl(+)Escherichia region as a persistent exogenote (partial diploids) were of intermediate virulence. These partial diploids may return to complete virulence by elimination of the Escherichia chromosomal fragment or become avirulent by incorporation of this fragment.