SEX IN BACTERIA— GF.NF.TIC STUDIES 1 3 



The next Step in the cvokition of our concepts of bacterial ge- 

 netics was the experimental application of the Neiirospora techniques 

 to the production of biochemical mutants in these simpler organisms. 

 The first nutritionally deficient (auxotrophic) mutants were pro- 

 duced in 1944 by x-ray treatment of Escherichia coli and Acetobacter 

 vielanogemim (Gray and Tatum, 1944; Roepke et al., 1944). Sub- 

 sequently, auxotrophic mutants have been obtained in almost every 

 species of bacteria investigated (see Tatum, 1946; Tatum and Perkins, 

 1950), In addition to mutations to growth-factor dependence and 

 reverse mutation to growth-factor independence, other types of 

 mutant characters have been obtained, thoroughly investigated, and 

 proved extremely valuable. These include such characters as virus 

 resistance, antibiotic resistance, and capacity for sugar utilization. 



The first auxotrophic mutants were obtained by the tedious and 

 laborious process of plating out irradiated cells on fully supplemented 

 medium, and then isolating individual colonies which were subse- 

 quently examined for failure to grow in the simple synthetic medium 

 adequate for the original stock (minimal medium). The specific 

 requirements of these deficient clones were then determined by sys- 

 tematic supplementation of the minimal medium with known growth- 

 factors. 



Later improvements in techniques have eliminated much of the 

 labor involved in isolating and testing mutants of bacteria, particu- 

 larly E. coli. These include (cf. Lederberg 1950, 1951a) the layer- 

 plate technique in which only presumptive mutants are isolated for 

 further testing, and the penicilhn method in which non-mutants are 

 actively eliminated, leaving mutant cells which form colonies after 

 removal of the antibiotic and suitable supplementation of the medium. 

 A further modification of this method, using solid medium and peni- 

 cillinase, permits easy isolation of any desired type of auxotroph 

 (Adelberg and Myers, 1952), and the isolation and testing of strains 

 has been still further simplified by the replica plating method (Leder- 

 berg and Lederberg, 1952) which in a single operation permits trans- 

 ferring all colonies on a plate to a number of other plates with dif- 

 ferent supplements. 



By all available criteria it is now generally accepted that most, if 

 not all, characteristics of bacteria are controlled by hereditary units, 

 and that these hereditary units in bacteria are analogous with genes 

 in classically sexual organisms in the independence and randomness 



