VIRULENCE OFESCHERICHIA-SHIGELLAGENETIC HYBRIDS FOR THE GUINEA PIG

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Abstract
Falkow, Stanley(Walter Reed Army Institute of Research, Washington, D.C.), H.Schneider, L. S. Baron, and S. B. Formal. Virulence ofEscherichia-Shigellahybrids for the guinea pig. J. Bacteriol.86:1251–1258. 1963.—Genetic recombination studies between donorEscherichia coliand recipientShigella flexneri2a strains were employed to examine alterations in the virulence ofShigellahybrids for guinea pigs. The genetic studies indicated that several chromosomal regions ofE. coliandS. flexneriare grossly homologous. The frequency of recombination betweenE. coliandShigellawas decreased, however, in comparison withE. coli×E. colimatings. Moreover, the predominantShigellahybrid classes acquired only the selected genetic marker, and extensive transfer of theEscherichiagenome 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 therha+andxyl+genes, which was essential for virulence. However, hybrids which had received theE. colipili antigen andfuc+-nic+determinants exhibited an intermediate virulence. Hybrids carrying a full complement ofShigellagenes and therha+-xyl+Escherichiaregion as a persistent exogenote (partial diploids) were of intermediate virulence. These partial diploids may return to complete virulence by elimination of theEscherichiachromosomal fragment or become avirulent by incorporation of this fragment.