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Juegative, Downloaded by Guest on September 26, 2021 VOL 976 GENETICS: BARON, CAREY, AND SPILMAN hoc. N. A. S. 27 Libby, W. F., "Radioactive Strontium Fallout," these PROCEEDINGS, 42, 365-390 (1956). 28 Libby, W. F., "Current Research Findings on Radioactive Fallout," these PROCEEDINGS, 42, 945-962 (1956). 29 Libby, W. F., "Radioactive Fallout," these PROCEEDINGS, 43, 758-775 (1957). 30 Public Hearings on Fallout from Nuclear Weapons Tests, Testimony before the Special Com- mittee on Atomic Energy, Special Subcommittee on Radiation, Joint Committee on Atomic En- ergy, Special Subcommittee on Radiation, Joint Committee on Atomic Energy, May 5-8, 1959. U.S. Government Printing Office (in press). 31 Daily Record of Fission Product Activity Collected by Air Filtration, Naval Research Labora- tory, Washington, D.C. GENETIC RECOMBINA TION BETWEEN ESCHERICHIA COLI AND SALMONELLA TYPHIMURIUM* BY L. S. BARON, W. F. CAREY, AND W. M. SPILMAN DIVISION OF IMMUNOLOGY, WALTER REED ARMY INSTITUTE OF RESEARCH, WASHINGTON, D.C. Communicated by Joshua Lederberg, May 12, 1959 Introduction.-The phenomenon of genetic recombination by sexual mating in bacteria, described by Tatum and Lederberg' has been investigated intensively by Lederberg et al.2 and many other workers. These classical studies were carried out almost entirely with the K-12 strain of Escherichia coli, although comparable re- sults were obtained with a small number of other strains of E. coli.1 4 In further studies, Cavalli5 found that the frequency of recombination could be greatly increased by using a highly fertile mutant of the K-12 strain. Additional high frequency of recombination (Hfr) strains of K-12 have been isolated in a number of laboratories.6'7 The concept of compatibility among certain K-12 derivatives has led to a detailed description of the fertility factor (F), so that it is now understood that mating consists of a unilateral contribution of genetic material from the Hfr or the F+ strain to the recipient Hfr, F- or F+ organism.8-1' In an attempt to demonstrate recombination between diverse species of bacteria, Zinder and Lederberg'2 discovered another mode of genetic transfer involving bacteriophage particles as vectors of genetic material. Intensive investigation of this phenomenon, referred to as genetic transduction, was undertaken by Stocker et al.,'3 Lederberg and Edwards,14 and Zinder,'5 using the phage PLT-22. Studies of additional bacterial viruses competent in transduction have been reported by Baron et al.,"6 Spilman et al.,"7 Sakai and Iseki,18 Lennox,'9 and Morse et al.20 Many of these results have dealt with the phage-mediated transfer of genetic ma- terial between different species of bacteria and even organisms classified in different genera have been altered by a suitable phage. 9 Genetic recombination between bacteria of different genera, however, has only recently been reported by Luria and Burrous2' with the demonstration of mating between E. coli K-12 and various species of Shigella. The results of these workers have indicated the presence of the same fertility system in Shigella species as is known to exist in the E. coli mating strains. Attempts at recombination of E. coli with Salmonella species by a number of workers, however, have been uniformly juegative, Downloaded by guest on September 26, 2021 VOL. 45, 1959 GENETICS: BARON, CAREY, AND SPILMAN 977 It is the purpose of this communication to describe experiments which involve the genetic recombination of E. coli and Salmonella typhimurium at high fre- quency. Certain of the results which have been obtained more recently suggest a generalized ability of other Salmonella species to mate with Hfr strain of E. coli at a much lower frequency. Preliminary data concerning this ability in other Salmonella species have been reported by Baron et al.22 and these results will be de- tailed in a future communication. Materials and Methods.-Cultures: A number of previously described derivatives of E. coli strain K-12 were employed; W1895 (Hfr), W-6 (F+), and W1485 were ob- tained from Dr. P. D. Skaar. S. typhimurium strain TM-9 was from the culture collection of the Walter Reed Army Institute of Research, as were other Salmonella species employed in some experiments. These strains were originally obtained from Dr. P. R. Edwards. Isolation of streptomycin resistant mutants (Sr) was accomplished by plating dense suspensions of streptomycin sensitive cells (S') on meat extract agar contain- ing 600 micrograms per ml of streptomycin sulfate (Abbott or Pfizer). Phages: The T series of coliphages was obtained from Dr. G. Bertani and were propagated on E. coli B and other host strains; phage PLT-22 was received from Dr. J. Lederberg and propagated on a number of suitable host strains of Salmonella. Phage sensitivity was checked by cross-streaking the phage preparations with the bacterial suspension to be tested on meat extract agar. Media: Meat extract agar was made to contain: Meat extract 3 g, peptone 10 g, NaCl 5 g, agar 20 g, and distilled water 1,000 ml. Minimal medium consisted of a separately autoclaved Difco Noble agar base made up double strength (14 g/400 ml water), to which was added 300 ml of phys- iological saline, 40 ml of a solution of KH2PO4 (60 g/l) and K2HPO4 (140 g/l), 40 ml of a solution of (NH4)2SO4 (20 g/l) and MgS04 7H20 (2 g/l), and 10 ml of a 20 per cent solution of the appropriate carbohydrate. The carbohydrates employed (lactose, l-arabinose, l-rhamnose, etc.) were Pfanstiehl products which were steri- lized by filtration. Eosin-Methylene Blue agar was made following the usual formula (Difco), but excluding lactose; in addition, the Baltimore Biological Laboratories kindly sup- plied a preparation of EMB agar which did not contain lactose. Where required, 20 ml of a 20 per cent solution of the appropriate sugar was added to each liter of this medium. Minimal EMB agar was prepared by the addition of the necessary amounts of the dyes to the minimal medium described above. EMB streptomycin agar contained 600 micrograms of streptomycin sulfate per ml. The Difco products, Penassay broth, Nutrient agar, Bacto-agar, and Phenol red broth were employed for routine cultivation, transfer, and testing of the strains. Antisera: Sera to E. coli, Salmonella species, and recombinants were prepared in rabbits by a series of injections of approximately 109 living cells of these organ- isms, or by injections of suspensions of organisms which were heat-killed at 100 C for two hours. Shigella hybrid F22 (from Shigella x coli) and antiserum to this strain2' were very kindly supplied by Dr. S. E. Luria. Regults.-A number of species of Salmonella were tested for their ability to mate with the Hfr or the F+ strain of E. coli K-12 in preliminary experiments. The species of SalmQnqlla jslected consisted of representative serotypes typical of Downloaded by guest on September 26, 2021 978 GENETICS: BARON, CAREY, AND SPILMAN PROC. N. A. S. the various groups of the Kaufmann-White diagnostic scheme. No attempt was made at this time to undertake a complete screening of the approximately four hundred or more characterized serotypes of the genus Salmonella, or of the huge number of isolates of any one serotype which were available. The cultures of E. coli and Salmonella were grown together in Penassay broth for approximately 24 hours at 37 C without aeration, centrifuged and washed three times with physiological saline, and then plated on minimal agar plates containing lactose (Lac) as the sole carbon source. The E. coli Hfr and F+ strains require methionine (M-) for growth, while an important criterion for classification of organisms in the genus Salmonella is their inability to ferment or utilize lactose. The minimal lactose (ML) agar medium was employed since it would fail to support the growth of either parental mating strain, but could detect M+ Lac+ progeny. For providing a control, the organisms also were grown separately; all the strains tested were stable lactose negatives (Lac-) and reversions to methionine inde- pendence by the E. coli strains were observed only rarely in control platings. In the initial experiment, a smooth culture of S. typhimurium strain TM-9, with the antigenic structure IV V XII: i, 1,2 mated with the Hfr strain of E. coli at low frequency; recombination of this strain could not be detected with the E. coli F+ strain. The E. coli Hfr would represent the best available strain for demon- stration of lactose positive (Lac+) recombinants since it has been characterized by its high frequency ability for the lactose factor.' Failure to observe recombi- nation using the F+ strain is to be expected when the frequency of mating with the Hfr strain is low. The colonies which appeared on the ML plates in the mating with the E. coli Hfr were examined and found to be Lac+ forms exhibiting the antigenic characteristics of the S. typhimurium parent strain. In further experiments, the plating procedure of mixing washed suspensions of the mating strains directly on ML agar or other selective media as described by Luria and Burrous2" was used. In agreement with their observations, frequencies of recombination were only slightly higher when parent strains were incubated together for a short time prior to plating. The frequency of recombination of S. typhimurium strain TM-9 with the E. coli Hfr strain (expressed as the ratio of recombinants to the number of Hfr parent cells) was approximately 4 X 10-8 using the procedure of mixing the strains di- rectly on ML agar. About 2 X 109 cells of the recipient parent strain were em- ployed usually. Frequencies of this low order have been observed with a number of other species of Salmonella by Baron et al.22 For purposes of convenience in selection and purification of recombinants, strep- tomycin-resistant variants of strain TM-9 were obtained as described earlier.
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