Prevalence of Escherichia coli strains exhibiting genetic recombination.

The first bacterium to be tested by an efficient selective method for the occurrence of genetic recombination was strain K-12 of Bsche~icl~ia coli. Experiments with auxotroph mutants of this strain promptly gave conclusive, positive evidence of genetic exchanges between 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 auxotroph strains from the National Type Culture Collection (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 resembles 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 following 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 wildtype strain K-12 in these markers: polyauxotrophy for threonine, leueine, thiamin; resistance to streptomycin and to bacteriophage Tl; failure to ferment lactose, maltose, mannitol, xylose, galactose, or Larabinose. 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 useful in detecting recombination between W-1177 and new strains to be screened. Heavy inocula of W-1177 and of the propositus are mixed in a complete broth tube, and incubated for 6-24 hr. The mixed culture is then harvested, and the washed cells are plated on a minimal agar medium containing lOO-1,000 ug/ml streptomycin. The minimal agar selects prototroph cells; thr streptomycin selects S’. The minimal streptomycin agar thus permits the growth only of T + L + B, + 6” colonies and suppresses the two parents. This assortment of c*harnc+tc~rs can arise either by recombinntiorr, or l)p mntiltifx~ of thr propositus from S” to S’.