APPENDIX 493 



TABLE XXIV 

 One-Step Mutants of Strain B of E. coli 



" Mutation rate/bacterium/generation. 



are resistant. However, this is not the only mechanism by which 

 bacteria can become resistant. . . . 



Strain B of E. coli and most of its phage-resistant mutants 

 grow abundantly on a chemically defined medium free from or- 

 ganic compounds other than an energy source such as glucose or 

 lactic acid (p. 446). However, E. H. Anderson (1946) discov- 

 ered that B/1 strains were consistently unable to grow in the 

 basal medium but would grow if tryptophan were added ; also 

 that certain B/3,4,7 strains were unable to grow unless proline 

 was present. The significance of this coupling of a nutritional 

 requirement with phage resistance is not clear. 



The mutations of strain B are additive; i.e., strain B can mu- 

 tate to B/1,5, which can mutate to B/1, 5/6, etc. Moreover, the 

 rate of mutation to a given resistance pattern is usually independ- 

 ent of previous mutations. B/1,5 mutates to B/1, 5/6 at the same 

 rate that B mutates to B/6. Also, the nutritional deficiencies are 

 additive; i.e., B/1, try ptophanless, can mutate to B/l,trypto- 

 phanless/3,4,7, prolineless which requires both tryptophan and 

 proline for growth. 



The phages of the T group are properly called coli-dysentery 

 phages since they attack various strains of shigella as well as E. 

 coli. Strains of Shigella flexneri I-VI, Boyd I-H, alkalescens, 

 ambigua, shiga, and sonnei were all susceptible to 1 or more 



