BACTERIOPHAGE GENETICS 283 



probably not more than ten or twenty. However, thousands of different 

 mutations have been used in the various genetic studies carried out with this 

 organism. The many mutations are separately arising inherited alterations 

 which affect one of this small number of observable characteristics. 



Most of the detailed genetic analyses have been done with the T phages, 

 which are active on Escherichia coli, and in particular the T-even members 

 of this group. The wild or standard type of these phages is defined as the 

 type which was first isolated for experimental purposes (Demerec and Fano, 

 1945), and strain B of E. coli has most commonly been used as indicator for 

 them. The wild-type phage produces a plaque of 1 to 2 mm. in diameter, 

 with a small clear center in which all the bacteria are destroyed and a wider 

 region in which the destruction of the bacterial layer is only partial and 

 gradually diminishes to zero. If a large number of such plaques are exam- 

 ined, one finds that approximately one in ten thousand are larger and have a 

 sharp edge delimiting the area of bacterial clearing. If these altered plaques 

 are picked and diluted, they are found to contain about 10'' particles, all of 

 which produce the same altered plaques. The phages which produce these 

 altered plaques are called "r" (for rapidly lysing) mutants (Hershey, 1946a,b) ; 

 their physiology is discussed in Chapter 7 in coimection with the phenomenon 

 of lysis inhibition (Doermann, 1948). Using strain B as indicator, only one 

 type of r mutant can be distinguished by the appearance of its plaque; 

 however, if different strains of bacteria are used, three different classes of r 

 mutants can be recognized (Benzer, 1957) by their appearance and by the 

 fact that some of them fail to produce any plaques on one or more of the 

 different indicators. 



Another set of directly observable mutations are the tus (turbids) used by 

 Doermami and Hill (1953) in studies with the phage T4. The plaques produced 

 by these phages have a turbid ring like the r"*", but the outer edge of the ring 

 is sharp and distinct. Under the proper conditions of plating, one can de- 

 termine by direct observation if a plaque was produced by a phage of the 

 type rtu, rtu^, r^tu, or r^tu^. In this and in the discussion of other mutants 

 we shall follow the standard genetic notation and use the symbol /•+ to mean 

 "like the wild type with respect to r" and tw^ to mean "hke the wild type 

 with respect to turbid." 



If certam indicator dyes are added to the agar, some phage mutants wiU 

 produce plaques in which there is a material which affects the color of the 

 dye. These "color mutants" of the phage Tl have been used to great advantage 

 and with considerable esthetic appeal by Bresch (1953) in studies of Tl 

 genetics. And finally, to complete the fist of directly observable plaque-type 

 mutants of the T phages, there is the minute (m) mutation (Hershey and 

 Rotman, 1949), which produces smaU plaques when it occurs in T2 or T4. 

 It should be kept in mind that this is not in any sense a complete fist of the 



