328 BACTERIOPHAGES 



Partial exclusion of the second kind was demonstrated by 

 Dulbecco (1952b). He infected bacteria with T2 and reinfected 

 them after various times with an r mutant of T2. (This pair 

 does not exhibit partial exclusion of the first kind.) He found 

 that the infected bacteria very quickly develop the ability to 

 exclude the superinfecting phage. The fraction of mixed yielders 

 falls to 0.5 after an interval of one minute and to 0.2 after two 

 minutes. It does not develop in infected bacteria that are starved 

 or whose metabolism is temporarily inhibited by cyanide. It 

 does develop in bacteria "infected" with ultraviolet killed T2. 

 In this instance the superinfecting phage need not be geneti- 

 cally marked. 



This excluding mechanism is partial in two senses. First, 

 some bacteria exclude and some do not. Second, some phage 

 particles infecting a given bacterium are excluded and others 

 are not. This was shown by Visconti (1953) who found that at 

 very high multiplicities of superinfection, nearly all the bacteria 

 become mixed yielders again. 



Partial exclusion of the second kind can also be demonstrated 

 by chemical methods (Lesley, French, Graham, and van Rooyen 

 1951; Graham, 1953). Half of the DNA of the excluded 

 phage particles is quickly split into acid-soluble materials and 

 excreted into the culture medium. This effect develops within 

 two minutes, just as the genetic exclusion does. The phe- 

 nomenon is seen after primary infection with T2, T4, T6, or T5, 

 including irradiated T2, but not after primary infection with 

 Tl, T3, or T7. It occurs on secondary infection with T2, 

 but not with Tl or T7. The breakdown is not the cause of the 

 exclusion, because if the bacterial deoxyribonuclease is inhibited, 

 breakdown fails but exclusion persists (French, Graham, Lesley, 

 and Van Rooyen, 1952; Hershey, Garen, Fraser, and Hudis, 

 1954). The limitation of the breakdown to 50 per cent of the 

 DNA of the superinfecting phage is probably due to the fact 

 that only half of the superinfecting particles inject (Hershey, 

 Garen, Fraser, and Hudis, 1954). 



Bacteria infected with T2 also develop resistance to lysis from 



