170 BACTERIOPHAGES 



example, the addition of penicillin (Krueger, Cohn, Smith, and 

 McGuire, 1948) or aureomycin (Altenbern, 1953) shortens the 

 latent period. Damage to phage particles caused by ultraviolet 

 light can result in a prolonged latent period for those phage par- 

 ticles surviving the irradiation (Luria, 1944). 



A phenomenon which has a marked effect on the duration 

 of the latent period is lysis inhibition (Doermann, 1948a), which 

 occurs with phages in the T2-C16 serological group. If, 

 throughout the latent period, an infected bacterium is con- 

 tinuously reinfected with phage, bacterial lysis may be delayed 

 for an hour or longer. The m_echanism of lysis inhibition is not 

 known. The ability to produce this phenomenon may be lost by 

 mutation of the phage to the so-called r (rapid lysis) form. 

 However, a mutant which is r type on one host may still produce 

 lysis inhibition on a different host (Benzer, 1957). Thus, lysis 

 inhibition is controlled by both phage and host. 



The latent period is insensitive to the number of phage par- 

 ticles with which the host cell has been infected (Delbriick and 

 Luria, 1942). This result was originally interpreted as indicat- 

 ing that only one of the adsorbed phage particles participated in 

 the infectious process. However, later work with genetically 

 marked phage particles indicated that as many as 10 particles 

 might successfully infect a single bacterial cell (Dulbecco, 1949b). 



Although the minimum latent period is a reproducible charac- 

 teristic of a given phage-host system, the precise times of lysis of 

 the individual cells are far from identical. There may be a two- 

 fold difference between the latent periods of individual cells. 



Adams and Wasserman (1956) described a probit method for 

 analyzing the variation in latent periods for individual cells. 

 They show that the method permits one to recognize heteroge- 

 neous populations of bacteria or phage arising from experimental 

 treatments of either one. 



3. Burst Size 



The burst size is the number of phage particles released per in- 

 fected bacterium. Its average value is determined by the one 



