VIRUS-INFECTED CELLS 243 



or that of other normal cytoplasmic particles, let alone vary their number 

 in a controlled way. On the other hand, we can enumerate very pre- 

 cisely the particles of certain bacterial viruses (bacteriophages), and 

 infect sensitive cells with known and carefully controlled numbers of 

 these particles. Let us infect the cells of a homogeneous bacterial popu- 

 lation with known numbers of a certain virus, so that each cell will con- 

 tain in average 1, 2, • • • n particles. Each infected cell represents an 

 infective center in the sense that, transferred to another appropriate 

 medium, it will carry and propagate the virus. By radiation, the in- 

 fective power of this cell can be suppressed by destroying the intra- 

 cellular virus; it has been shown that the infective power persists as 

 long as a single intact virus remains in the cell (21). In this case, 

 2 = 1. If we consider as a lesion the suppression of the infectivity of 

 each cell, we can establish experimentally the "survival curve," showing 

 for every dose of radiation the proportion of cells retaining their in- 

 fectivity, that is, still containing at least one intact virus capable of 

 multiphcation. 



We are dealing with a schematic case of a lesion in which all {z = 1) 

 the units of a certain intracellular element, whose number n can be 

 set between 1 and about 30, must be affected. We have evidence that 

 each of these particles is inactivated by a single quantum of radiant 

 energy, either ultraviolet photon (18) or ionization (32). The number 

 of effective quanta required to injure each cell is thus in principle equal 

 to n. Classical calculation shows that in theory, that is to say, assuming 

 a discontinuous mode of action of radiation, the lesion curves should 

 follow general formula 1 (Fig. 1) : 



y=l-{\- e-"''r (1) 



where y is the proportion of uninjured cells. 



D is the dose in number of quanta absorbed per unit volume. 



a is the quantum yield. 



n is the initial number of particles. 



Experiments (21, 15) have given results in agreement with these theo- 

 retical predictions. Bacteria {E. coli, strain B) in known number were 

 mixed in liquid medium with a known number of particles of bacterio- 

 phage T2 so that the average number n of phage particles absorbed by 

 each bacterium was controlled precisely. Under conditions precluding 

 any intracellular multiplication of the virus, infected bacteria were ir- 

 radiated either with ultraviolet or x-rays, and the lesion curves were 

 drawn for each value of n. It was found (Fig. 2) that they corresponded 

 satisfactorily with the theoretical curves of Fig. 1. 



