222 



suits of such experiments with Sal- 

 monella typhijjnirium and other Sal- 

 mo7iella serotypes. The mechanism of 

 genetic exchange found in these ex- 

 periments differs from sexual recom- 

 bination in E. coli in many respects so 

 as to warrant a new descriptive term, 

 transduction. 



MATERIALS AND METHODS 



Most of the strains of S. typhimii- 

 riiim were provided by Lilleengen 

 (1948) as representative of his 21 

 "phage types", LT-1 through LT-22. 

 Most if not all strains of 5. typh'mni- 

 riimi are lysogenic (Boyd, 1950), and 

 these have provided 12 lines of 

 bacteriophage. Other cultures were 

 obtained from F. KaufTmann, E. K. 

 Borman, and P. R. Edwards. All cul- 

 tures were maintained on nutrient agar 

 slants. 



Specific growth factor dependent 

 mutants (auxotrophs) were obtained 

 from ultraviolet irradiated cell suspen- 

 sions subjected to the penicillin method 

 for selective isolation (Davis, 1950^; 

 Lederberg and Zinder, 1948). Similar 

 mutants have been obtained in Sal- 

 monella by Plough et al. (1951) and 

 Bacon et al. (1951). Other methods for 

 the isolation and characterization of 

 auxotrophic and fermentation mutants 

 have been documented elsewhere 

 (Lederberg, 1950; Lederberg and Led- 

 erberg, 1952). Streptomycin resistant 

 mutants were selected by plating 

 dense, unirradiated cell suspensions 

 into agar containing 500 mg per L of 

 dihvdrostreptomycin. 



"Complete" indicator medium 

 (EMB) was made up from the same 

 formula as for E. coli (Lederberg, 

 1950). The defined eosin methylene 

 blue medium ("EML agar") contained 



University of Wisconsin as a dissertation for 

 the degree of Doctor of Philosophy. His 

 present address is Rockefeller Institute for 

 Medical Research, New York, New York. 



ZINDER AND LEDERBERG 



in g per L: sodium lactate, 2.5; 

 (NHi)oS04, 5; NaCl, 1; MgS04, 1; 

 K2HPO4, 2; methylene blue hydro- 

 chloride, 0.05; eosin Y, 0.3; and agar, 

 15. Difco products, penassay broth, 

 and nutrient agar, were employed as 

 "complete" media. 



Unless otherwise stated, all cultures 

 were incubated at 37C, and plates 

 were scored after 24 and 48 hours. 



EXPERIMENTAL RESULTS 



Direct crosses: platings of mixed 

 cultures. In E. coli., recombinants 

 were detected selectively by plating 

 various auxotrophs together on mini- 

 mal agar. Both parents are suppressed 

 on this medium and, barring various 

 experimental errors, colony formation 

 is confined to prototrophic recom- 

 binant cells. Such errors can be de- 

 tected by appropriate controls but are 

 best mitigated by the use of double 

 nutritional mutants (diauxotrophs). 

 These are obtained by the iterated iso- 

 lation of mutants in previously estab- 

 lished auxotroph lines. 



One of Lilleengen's strains was 

 refractory to our techniques of mutant 

 isolation. Two-step mutants with mu- 

 tually complementary nutritional re- 

 quirements were prepared from each 

 of the remaining twenty types. Of the 

 two hundred possible pairwise com- 

 binations, including "selfed" crosses, 

 one hundred were tested. Each com- 

 bination was studied by mixing and 

 plating 10^ washed cells of the two 

 parents on a minimal agar plate. Fif- 

 teen mixture plates and five control 

 plates for each parent by itself were 

 inoculated in each test. Fifteen com- 

 binations yielded prototrophs in con- 

 trast to barren controls. Strain LA-22 

 was the most "fertile", especially with 

 LA-2 (see table 1). This cross yielded 

 about one prototroph per hundred 

 thousand parental cells plated. Crosses 

 in which LA-22 was not involved gave 



