RADIOBIOLOGY OF BACTERIOPHAGE 



359 



number of joliage particles. Equation (5) can be modified (Luria and Dulbecco, 

 1949) to describe theoretically the probabihty of phage production by the 

 multiply infected members of a bacterial population upon which phage 

 have been randomly (Poisson) adsorbed.^ 



S ^T fl - (1 - ^"'"^'^^ 



1 - (a; + l)e- 





12 16 20 



UV dose, r 



Fig. 3. Multiplicity reactivation of ultraviolet-irradiated Vi-Phage II of Salmonella 

 typhi. Values for the survival of multicomplexes {iv) plotted against the dose in phage- 

 lethal hits (r). Ihe solid line gives the survival of the phage and the other curves are 

 those of log y as a function of r for % = 4 and for x as indicated at the foot of each curve 

 [see Equation (7)]. The figure is from Bernstein (1957). 



For a given type of phage, the number of imits, n, is presumed constant, 

 so that y is dependent on two variables, the average multiplicity of infection, 

 X, and the average number of hits per phage, r. At doses so high that essenti- 

 ally no productive cells have more than the minimal number of units required 

 to make phage, y depends on dose in the same way as does the survival of a 

 single phage particle. That is 



yr^^ = Be-^ (8) 



* The adequacy of the assumption of a Poisson distribution of phages upon cells has 

 been challenged (Cairns and "Watson, 1956) on the grovmds that one cannot neglect the 

 small number of cells in the culture "which are considerably larger than average. Harm 

 (1956), however, found identical results in multiplicity reactivation experiments, using 

 either standard cultvu-es or cultures in which the usual variability in cell size had been 

 greatly reduced by differential filtration. 



