STAGES IN PHAGE MULTIPLICATION 169 



2. The Latent Period 



After adsorption of phage to the host cell, a period of time 

 elapses before the cell lyses, liberating phage progeny. This 

 time interval is known as the latent period and is measured by a 

 one-step growth experiment (Chapter II). In such an experiment, 

 not all the infected cells lyse at the same time. However, if the 

 plaque count is plotted on a logarithmic scale as a function of the 

 time of sampling, the rising portion of the curve is linear (Adams, 

 1949b; Maal0e, 1950; Barry and Goebel, 1951; Doermann, 

 1952; Bentzon, Maal0e, and Rasch, 1952). On such a plot, 

 the intersection of the line with the baseline count of infected bac- 

 teria provides a suitable working definition of the miniinum la- 

 tent period. 



The latent period depends on the phage and host strains, and 

 also on the environmental conditions. Different phages growing 

 on the same bacterial strain may have widely different latent 

 periods (Delbriick, 1946b). Even closely related phages such as 

 T2, T4, and T6 (Delbriick, 1946b) and T5 and PB (Adams, 

 1951b), may have quite distinct latent periods on the same host. 

 In the case of T5 and PB, Adams (1951b) showed that the latent 

 period of each phage may be separated from other characteris- 

 tics by genetic recombination. The influence of different hosts 

 on the latent period of the same phage has been reported by 

 Barry and Goebel (1951). The latent period is strongly affected 

 by the physiological state of the host cell, as demonstrated by 

 Delbruck (1940a) and by Heden (1951). Delbruck (1946b) re- 

 ported that the latent periods for phages Tl, T2, and T7 were 

 the same in a synthetic medium as in broth, even though the 

 bacterial growth was much slower in the synthetic medium. 

 This is not true of all phage-host systems; Adams (1949b) found 

 a latent period for phage T5 of 55 minutes in synthetic medium 

 as compared with 40 minutes in broth. 



Temperature generally affects the latent period in a similar 

 manner to its effect on the generation time of the bacteria (Ellis 

 and Delbruck, 1939); at lower temperatures, both increase. 



Metabolic poisons are known to alter the latent period. For 



