An Hfr strain of E. coli K-12 has been shown to mate at low frequency with a number of strains of S. typhosa. The hybrids, selected as lactose positives, retained all other antigenic and biochemical manifestations of the S. typhosa parent. A Lac+ hybrid, S. typhosa strain 643L+ was remated with the E. coli Hfr and plated on minimal media containing L-arabinose, D-xylose, L-rhamnose, or L-fucose as the sole carbon sources. Hybrids of the remated strain appeared at high frequency on the plates containing L-arabinose, and could be detected at lower frequencies on plates containing D-xylose, L-rhamnose, and L-fucose. Those selected for D-xylose or L-rhamnose utilization possessed the attributes of segregating diploid heterozygotes being highly unstable and continually segregating a cultural form typical of the S. typhosa parent. The unstable type exhibited most of the biochemical characteristics of the E. coli parent including the ability to produce indol, and also reacted with antiserum to both the E. coli and S. typhosa parent strains, owing to the acquisition of a thermolabile antigen from the E. coli parent. The parent and hybrid strains were examined in detail for changes in patterns of phage susceptibility and virulence. Acquisition of susceptibility to a number of the T phages, a characteristic of the E. coli parent, was observed in one of the hybrid types. A decrease in virulence of the diploid hybrid form of S. typhosa in the mouse virulence test was found.

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