122 BENZER ET AL. 



and cannot be recovered by any known means. The bacterium is killed 

 in the sense that it fails to proliferate and cannot be made to do so by 

 any known means. The respiratory rate is not impaired, but it also 

 does not increase, as it would in the uninfected bacterium (Cohen and 

 Anderson, 1947; Monod and Wollman, 1947). The bacterium also has 

 lost its ability to form adaptive enzymes (Monod and Wollman, 1947). 

 A further characteristic feature of this stage is that mutual exclusion 

 has become estabhshed (see 31). One has to imagine that at this stage 

 the attacking virus and the bacterium have formed a functional unit 

 designed for the production of phage but not necessarily able to do so. 

 The cases in which progress gets arrested at this stage are the follow- 

 ing: 



20. Phage T5 in the absence of calcium (Adams, 1949). — In the 

 absence of calcium, this phage is adsorbed and kills the bacterium, but 

 no new phage is produced. If calcium is added later, lysis and phage 

 liberation take place 40 minutes after the addition of calcium. Forty 

 minutes is the normal latent period of this phage. The absence of cal- 

 cium seems to block the development at a very early stage. 



2 1 . Ultraviolet (UV ) treated phage is also arrested at the invasion 

 stage, i.e., such phage is adsorbed and kills the bacterium (Luria and 

 Delbriick, 1942) and excludes others (see 31). UV phage is of extreme 

 interest because of the various reactivation phenomena associated with 

 it, namely, photoreactivation described in 33, and multiplicity reactiva- 

 tion described in 34. 



When phage is exposed to the direct effect of X-rays approximately 

 1 out of 3 "hits" destroys the ability of the phage to kill the bacteria. 

 X-ray inactivated phage thus consists of two fractions: a "killing" frac- 

 tion which kills the bacteria following adsorption, and a "non-killing" 

 fraction which adsorbs without killing (Watson, 1950). 



V. Multiplication 



22. Burst size distribution — correlation with size of bacteria. — 

 The yield of virus from an individual bacterium can be determined. 

 It is called the burst size of the bacterium (see 12). One finds in all 

 cases that the burst sizes vary enormously, from a few up to around 

 1,000, with a very broad maximum (Delbriick, 1945b). A part of 

 this variability of the burst size may be due to variations in the sizes 

 of the bacteria. In fact, from observations in the dark field microscope 

 one does get the impression that larger bacteria liberate on the average 

 more particles than do small bacteria. However, the variations in burst 



