242 G. S. STENT 



conditions under which growth is allowed to proceed. The latent period, not 

 surprisingly, is longer at lower incubation temperatures. Ellis and Delbriick 

 (1939) fomid, for instance, that the latent period of their coliphage is 30 

 minutes at 37°, 60 minutes at 25°, and 180 minutes at 16°C. Maalife (1950), 

 who examined in more detail the prolongation of the latent period at lower 

 temperatures, observed that the temperature effect is not the same at different 

 points in the latent period, as if the successive stages of intracellular phage 

 multiplication possess very different temperature sensitivities. The nutrition 

 and physiological "age" of the bacterial host cell can also affect both latent 

 period and burst size. In a poor growth medium or in lag-phase cultures the 

 latent period is often longer and the burst size smaller than in bacteria grow- 

 ing rapidly on a favorable substrate (Delbriick, 1940; Heden, 1951). Latent 

 period and burst size, finally, differ widely for various types of bacteriophages 

 (cf. Delbriick, 1946) and may vary even for different mutants of the same 

 phage strain (Symonds, 1957), or for the same phage strain gTOwing on differ- 

 ent hosts (Barry and Goebel, 1951). The latent period, however, does not 

 appear to depend in any very striking way on the number of phage particles 

 with which each bacterial cell has been infected, since the first progeny phages 

 make their appearance at about the same time, whether the concentration 

 of infecting phages is much less than that of the host cells (in which case each 

 bacterium which is infected at all is infected with only a single particle), or 

 whether the concentration of infecting phages greatly exceeds that of the 

 host cells (in which case each bacterium is infected with many particles) 

 (EUis and Delbriick, 1939; Delbruck, 1940). 



B. Single Burst 

 The one-step growth experiment follows the mass behavior of thousands of 

 phage-infected bacteria. The latent period, therefore, is a minimum parameter 

 and reflects only the time required for lysis of those cells in which phage 

 development has progressed most rapidly, while the burst size reflects only 

 the average phage yield per ceU. Ellis and Delbriick (1939) also devised a 

 single-burst experiment, by means of which it is possible to study phage 

 development in individual infected bacteria. Immediately after infection, 

 numerous small aliquots of a high dilution of the culture of a one-step growth 

 experiment are placed into separate little tubes, so that each tube contains 

 on the average less than one infected bacterium. If the average number of 

 infected bacteria per tube is m, then the fraction of tubes, p^, having received 

 r infected cells is given by the Poisson law 



^^^-e- (1) 



Hence, provided that m is sufficiently less than one, most of those tubes which 

 have any infected bacteria will have only one. The tubes are then incubated 



