Sexuality in bacteria (1), now considered to be a unilateral conjugation (2, 3), has been studied in Escherichia coli b y m a n y workers. Luria and Burrous (4) have extended this form of genetic recombination to bacteria classified in different genera with their description of mating between E. coli and Shigella species. Recently, successful crosses between E. coli and Salmonella typhimurium were reported b y Baron eta/. (5). These studies dealt with the isolation of a streptomycin-resistant m u t a n t of S. typhimurium which acted as a high frequency recipient strain in matings with E. coli. Further studies b y Baron et al. (6) Materials and MethodsCultures.--The strains of S. typhosa employed were obtained from the culture collection at the Waiter Reed Army Institute of Research. A high frequency of recombination (Hfr) derivative of E. ¢otl K-12, strain W-1895, was obtained from Dr. P. D. Skaar, and a D-arabinose utilizing (v-Ara) mutant of this organism was isolated and employed in these studies.Media.--Meat extract agar, minimal media, EMB agar, and other culture media were used as described previously (5).Antisera.--Rabbit antisera were prepared against E. coU, S. typhosa, and selected hybrids by a series of intravenous injections of 109 viable or heat-killed (100°C. for 2 hours) cells. Absorbed sera were prepared as described by Edwards and Ewing (7).Phages.--The Vi typing phages were obtained from Dr. Cora Gunther; the procedures used for propagating these phages have been described by Baron et aL (8). The T series of bacteriophages were received from Dr. G. Bertani.Recombination Procedures.--For the detection of strains fertile with E. coli K-12, Hfr strain W-1895, the various cultures of S. typhosa which were to be tested as recipients (females) were spread with the E. coli culture as the donor (male) strain at a ratio of twenty recipient cells to one donor cell on an appropriate selective medium. The E. coli culture was grown in

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A number of species of Salmonella were fertile at low frequency as recipients in crosses with Escherichia coli, as evidenced by the isolation of lactosepositive hybrids possessing the Salmonella antigenic structure. A hybrid from an initial mating, when crossed again with E. coli, behaved then as a high-frequency recipient strain.

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