332 



CHAPTER 36 



FIGURE 36-3. Discrete clones, grow- 

 ing on agar nutrient in a petri dish, 

 obtained by plating a dilute culture of 

 bacteria. 



FIGURE 36-4. Separate bacterial clones 

 obtained by the streaking method. 

 {Courtesy of N. E. Melechen.) 



restricted to morphological variation, since 

 it is not yet feasible or possible to make physi- 

 ological and biochemical studies on such a 

 microscopic scale. We can, however, make 

 use of the fact that, barring mutation, clones 

 are composed of genetically identical indi- 

 viduals. Genetically different clones may 

 show phenotypic differences in the size, shape, 

 and color of the colonies formed on agar. 

 Genetically different clones may also respond 

 differently to various dyes, drugs, and viruses. 

 Therefore, we can establish the genotype of a 

 single bacterium from the phenotype it pro- 

 duces as a clone, and we have already indi- 

 cated a number of clonal traits useful for this 

 purpose. 



E. coli, for example, is an easily cultured 

 organism. This is so because it can grow and 

 reproduce on a simple, chemically defined 

 food medium. Strains which grow on such 

 a basic, minimal medium may be considered 

 to hQ prototrophic, or wild-type, being capable 

 of synthesizing the numerous metabolic com- 

 ponents of the cell not supplied in the me- 

 dium. In this respect prototrophs of E. coli 

 or other bacteria are identical to wild-type 

 Neurospora which also grows on a minimal 

 medium (Chapter 32, pp. 281-282). It is no 

 surprise, then, that in bacteria, just as in 

 Neurospora, the richest source of mutants 

 comes from the study of the biochemical 

 variations which occur in different clones, 

 particularly those involving changes in nu- 

 tritional requirements. And, in fact, numer- 

 ous mutants have been obtained, arising either 

 spontaneously, or after treatment with physi- 

 cal or chemical mutagens, which, in order to 

 grow and reproduce, require the addition of 

 one or more of a variety of chemical sub- 

 stances to the basic medium. Thus, for ex- 

 ample, one strain of E. coli requires the addi- 

 tion of the amino acid threonine to the mini- 

 mal medium in order to grow, while another 

 strain requires the amino acid methionine. 

 Such nutritionally dependent strains, which 

 need a supplement to the basic food medium 



