284 C. LEVINTHAL 



possible alterations which could be observed as plaque-type mutants. It 

 seems likely that many others could be found if sufficient ingenuity and 

 effort were devoted to establishing plating conditions on which the altera- 

 tions could express themselves, 



B. Host- Range Mutants 



All the mutations mentioned so far are observable using E. coli strain B as 

 the plating bacterium. If different strains are used, they may enhance or 

 decrease the observable difference. However, there is another type of muta- 

 tion which can be observed if bacterial mutants are specially selected for 

 this purpose. By mixing a very large number of wild-type phage T2 and 

 many bacteria in a layer of nutrient agar, T2-resistant bacteria, designated 

 B/2, can be selected which will grow even in an excess of phage. If these 

 resistant bacteria are used as indicator, the wild-type phage will not produce 

 plaques; but if many wild-type phages are plated on them, it is possible to 

 select phage mutants which are able to grow and produce plaques on the 

 resistant bacteria. These host-range mutants, which are designated h, are 

 discussed more fuUy in the chapter on adsorption, since the genetic character 

 expresses itself by its effect on the ability of the phage to adsorb to the 

 resistant cells. Using mixed indicator, which contains both the sensitive and 

 resistant cells, one can determine by direct observation whether the phage 

 producing a particular plaque was A or A+. The h phage will produce a clear 

 plaque, since it can destroy both types of cells, while the 7i+ phage will 

 produce a plaque which is turbid because it is overgrown by the resistant 

 cells. This process of selecting resistant bacteria and using them to obtain 

 phage mutants with increased virulence can be repeated and has been used 

 by Baylor et al. (1957) to obtain two steps beyond h for the T2 system, and 

 by Fraser and Dulbecco (1953) to obtain a set of T3 mutants with numerous 

 levels of extended host range. 



Several of the systems of mutations wiU be discussed below in connection 

 with some of the special problems for which they have been particularly 

 useful. However, it is worth noting that the sets of mutations which are 

 observable depend very much on the particular type that was originally 

 selected as the standard. For example, if one had started the study of T2 

 genetics with an ^-type phage and the bacterium B, it is probable that the 

 B/2 and /i+ mutation would never have been obtained, since one needs the 

 mutation to B/2 in order to recognize the existence of an h^ phage as different 

 from the h and one needs the existence of the h^ phage to detect the presence 

 of a resistant bacterium. 



Each of the mutations discussed so far behaves as a single-step process. 

 However, one can obtain phages which differ from the wild type by several 

 mutations if particles mutant for one property are first selected and a large 



