296 BACTERIOPHAGES 



Phenotypic mixing thus differs from host-induced modifica- 

 tions with respect to the interacting units, the characters affected, 

 and the number of particles affected. It shows in a remarkable 

 way how two kinds of phage can cooperate in the formation of 

 a single particle. 



d. Other Phenotypic Modifications 



Agencies such as specific antiserum, radiations, heat, and 

 chemicals applied to extracellular phage particles produce, as 

 far as is known, exclusively phenotypic effects. They do not, at 

 any rate, produce typical mutations. Only a few examples are 

 of significance in the present context. 



Ralston and Krueger (1954) noted, as is commonly observed 

 with phages, that the rate of loss of infectivity of the staphylo- 

 coccal phage PI 4 at 59 ° C. varies with the host on which titra- 

 tions are made. As a result, the effect of heating mimics the 

 effect of the host-induced modification ; both produce the same 

 restriction of host range. This may or may not suggest a com- 

 mon point of action for heat and the host-induced modification, 

 but two items of phage lore are worth citing in this connection. 



On the one hand, antiserum and heating can produce similar 

 restrictions of host range (Kalmanson and Bronfenbrenner, 

 1942). Both antiserum and heating probably interfere with 

 adsorption or penetration (Chapter VIII; Streisinger and 

 Franklin, 1956). Thus the effects of heat are probably quite 

 different in principle from typical host-induced modifications. 

 Ralston and Krueger did not make the pertinent test of reacting 

 antiserum with their phage in its unrestricted host range modi- 

 fication. Neither did they test the ability of the restricted phage 

 produced by heating to adsorb to the refractory host. 



It is possible that heat-inactivated phages can sometimes 

 adsorb. Strongly heated preparations of T2 often show small 

 satellite plaques surrounding the few normal plaques appearing 

 on assay plates (Hershey, Franklin, and Streisinger, personal 

 communications). This suggests that some of the heated phage 

 particles can adsorb but fail to multiply unless helped out by 



