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Prevalence of Escherichia coli Strains Exhibiting Genetic Recombination

American Association for the Advancement of Science
Publication Date
  • Escherichia Coli
  • Recombination
  • Genetic
  • Medicine


Prevalence of Escherichia coli Strains Exhibiting Genetic Recombination Joshua Lederberg Department of Genetics,1 College of Agriculture, University of Wisconsin, Madison The first bacterium to be tested by an efficient selec- tive method for the occurrence of genetic recombina- tion was strain K-12 of Bsche~icl~ia coli. Experiments with auxotroph mutants of this strain promptly gave conclusive, positive evidence of genetic exchanges be- tween different mutant cells in mixed cultures (1, 2). However, subsequent attempts to obtain comparable results with a number of other strains used for genetic work were fruitless. Cavalli and Heslot (3) examined a number of auxo- troph strains from the National Type Culture Collec- tion (England) and found one that could be crossed with K-12. Unfortunately, this isolate has a complex nutrition, so far unanalyzed, which greatly hinders further work. In other characteristics it closely re- sembles K-12. It would be surprising if K-12, the first E. coli strain examined, should prove to be uniquely suitable for crossing experiments. Unfortunately, the method for testing fertility involved a good deal of work: it was necessary to prepare at least two nonoverlapping, double nutritional mutants from each strain. Despite improved techniques (4)) such a procedure is almost prohibitive for routine survey of new strains. The fol- lowing procedure was therefore put into effect for preliminary screening. A multiple marker strain, W-1177 (= 677-sr in [5] ) has been developed from K-12 by a long sequence of mutational steps. This strain differs from the wild- type strain K-12 in these markers: polyauxotrophy for threonine, leueine, thiamin; resistance to strepto- mycin and to bacteriophage Tl; failure to ferment lactose, maltose, mannitol, xylose, galactose, or L- arabinose. These may be symbolized as : T - L-B, - S’ V,’ Lnc - Ma2 - etc. Typical wild-type E. coli strains are Z’+ L + B, + 8”. These four markers are

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