350 RADIATION HIOLOOY 



the contrary, is now coiisidcred a rathor Kood .source of vacciiie.s (Web.ster 

 and Ca.sals, 1942; Leviiusoii el *//., I'.tll; Milzer et at., 1941; Mil/er and 

 Levinsoii, \\)V.)). As is the rule with inactive viruses, hirf^c; amounts of 

 irradiated virus must be used in vaccination .since there is no increase in 

 antigen l)y niultipHcation of virus in the host. It is po.ssihle that a certain 

 role in the immunity phenomena oh.served with inactive virus vaccines 

 may be played by the interference phenomena discus.sed in the next 

 section. 



4. IRRADIATED VIRUS IN MULTIPLE AND MIXED INFECTION 

 l-l. INTERFEREXCE PHENOMENA 



Under interference phenomena is included a complex jjjroup of phe- 

 nomena involving an alteration in the growth or manifestations of a virus 

 due to the presence in the same hc^st of more virus of the same or anotiier 

 type. The virus particles do not interact among them.selves in vitro, and 

 the interference phenomena are strictly cellular. Only with bacterio- 

 phage, however, have interference phenomena been analyzed at the 

 cellular level (Dell)rack, 1950). 



Mixed infection of a common host with two unrelated bacteriophages 

 results in mutual exclusion, only one virus type reproducing in any one 

 given cell. The excluded virus may exert a depressor effect on the yield 

 of \vinning virus. These exclusion phenomena are not exerted at adsorp- 

 tion but take place intraceUularly. Related viruses give incomplete 

 exclusion, which becomes less and less evident as the viruses become more 

 closely related; particles of two virus strains differing by one mutation 

 only do not exclude one another. Whenever exclusion fails, the total 

 yield of virus per cell is lower than the sum of the yields that each virus 

 would produce by it.self; the two viruses share the maximum potential 

 yield per cell. 



With irradiated phages the following rule is fairly well established: 

 Whenever a phage particle, after exposure to ultraviolet or X rays, can 

 still invade and kill the cell, it retains the exclusion power it had when 

 active; particles that are adsorbed but do not kill the host do not produce 

 exclusion (Luria and Delbruck, 1942; Watson, 1950). It is not known 

 whether an ultraviolet- or an X-ray-inactivated virus particle, if excluded, 

 can still exert a depressor effect on the yield of an active, heterologous, 

 excluding virus. Irradiated interfering phage excludes homologous 

 active phage if it reaches the bacterium several minutes earlier; otherwise, 

 exclusion fails, and the yield of active phage is normal (Luria and Dul- 

 becco, 1949). 



It is evident then that the interfering ability of inactivated phages is 

 related to their ability to kill the bacterial host. If, as seems likely, the 

 latter process results from the virus taking oxor and redirecting the syn- 



