292 BACTERIOPHAGES 



tion rather than a mutation, which would be expected to revert 

 with much lower frequency. Numerous examples of adaptive 

 host-controlled modifications are described in Chapter XXI, 

 together with additional evidence for their nonmutational origin. 

 The phenomenon has been observed in many phages by Ander- 

 son and Felix (1952), Ralston and Krueger (1952, 1954), 

 Fredericq (1950), N. Collins Bruce (cited by Luria, 1953b), 

 Bertani and Weigle (1953), Fredericq (1953), Garen and Zinder 

 (1955), Beumer and Beumer-.Jochmans (1955), and E. B. Collins 

 (1956). 



Only one example of a nonadaptive host-induced modification 

 is known (Luria and Human, 1952), but it was the first host- 

 induced modification to be recognized, and is exceptional also 

 because the modifying host (a particular strain of B/4) is a 

 mutant of the customary host (B). The nonadaptive character 

 of the restricted host range is indicated by the fact that T2 

 grown on B/4 loses its capacity to grow on B/4, but regains this 

 capacity on being returned to host B. 



The common characteristics of all these host-induced modifica- 

 tions may be listed as follows (Luria, 1953b). 



7. The modifications of the phage do not afTect its ability to 

 adsorb to its various bacterial hosts. The restricted growth on cer- 

 tain hosts is not due solely to failure to inject (Luria and Human, 

 1952; Garen and Zinder, 1955), thoughefficiency of injection has 

 not been measured in any instance. The abortive infection may 

 kill the refractory host, as with T2 and with certain Vi phages of 

 S. typhi, or not, as with P2 or P22. This difference probably 

 reflects general diff'erences in the properties of the phages, not 

 differences in the nature of the host-induced modification. 



2. The few particles of a restricted phage that manage to 

 grow in cells of the refractory host are not exceptional particles; 

 rather the productive bacteria are exceptional. This is indi- 

 cated by the fact that the proportion of productive bacteria varies 

 with the conditions under which the bacteria were grown before 

 infection and with treatments, such as ultraviolet irradiation, 

 applied to the bacteria before infection. Single bursts of phage 



