SYLLABUS 123 



size are certainly very much greater than the variations in bacterial 

 volume. 



23. Latent period. — The latent period between infection and lysis 

 also varies from bacterium to bacterium, but to a much lesser degree 

 than the burst size. The point that can be determined with greatest 

 convenience and accuracy is the minimum latent period, and this is 

 generally meant when we speak of the latent period. For the seven 

 phages of the T series the latent periods are given in Table I (p. 147). 

 These are the values obtained at 37°C, in nutrient broth, and with 

 bacteria in their phase of most rapid division. 



A very striking feature about the minimum latent period is that it 

 is the same in the case of multiple infection as in single infection (Del- 

 briick and Luria, 1942). This was first established in 1942 and has 

 since been confirmed in every well-studied case. A bacterium infected 

 with 10 phage particles does not seem to have a head start over one 

 infected with only one phage particle. Doermann has broken up bac- 

 teria prior to their natural term of lysis and has found that multiply 

 infected bacteria are very slightly ahead of singly infected bacteria 

 opened at the same time (see 27) . 



24. Influence of growth medium. — If the growth medium is 

 changed the latent period changes very little, even though the bacteria 

 themselves may grow much more slowly in the new medium. Thus, the 

 latent period of T2 in nutrient broth is 21 minutes. In this medium, 

 the bacteria divide every 19 minutes. In a synthetic glucose medium, 

 where the bacteria divide about every 40 minutes, the latent period of 

 T2 is still 21 minutes. It is generally found that the burst sizes are 

 smaller in simple media than in broth. One has to imagine that the 

 slow growth of the bacteria is due to the higher complexity of synthetic 

 operations required of a bacterium living in a simple medium. This 

 complexity limits also the rate of synthesis of material that is to go into 

 virus, thus decreasing the burst size, but the latent period is determined 

 by internal factors, independent of assimilatory procedures. 



25. Dependence of the latent period on the temperature. — A 

 change of temperature, in contrast to a change of mediiun, changes the 

 pace of all bacterial functions, not only the pace of assimilation. It is 

 therefore not surprising to find that a change of temperature affects also 

 the latent periods, roughly in proportion to the influence that tempera- 

 ture has on the division time of the uninfected bacteria (Ellis and Del- 

 briick, 1939)- 



26. Dependence of growth upon specific supplies. — Two facts 

 have been well established. 



