352 



RADIATION UIOI.OOY 



mixed witli scnsitiNc bacteria. Kor irradiated phase tlie residual titer is 

 minimum when the phaf^e is exposed to such an excess of bacteria that the 

 f2;reat majority of tlie infected l)acteria receive only one phage particle. 

 I'nder these conditions, it is possible to distinguish a fraction of "residual 

 active particles" and one of "inactive particles." The usual inactivation 

 curves are obtained in this manner. For some phages and for some types 

 of radiation, there is an apparent increase in activity under conditions of 

 "multiple infection" of bacteria with phage. This multiplicity reacti- 

 vation has been observed with the coli phages Tl, T2, T4, Tf), and T6 

 after ultra\iolet irradiation (Luria and Dull)ecco, 1949) and with T2, T4, 

 and T() also after exposure to X rays (Watson, 1950). To participate in 

 reactivation, an irradiated particle must retain its ability to kill the host 

 (see Sect. 3-2). 



Table 9-2. The Basic Observation in Multiplicity Reactivation of 



Bacteriophage 



(Modified from Luria and Dulbecco, 1949, Table 2.) 



Phage: T6r, 1.5 X 10'" units/ml exposed for 20 sec to ultraviolet germicidal lamp, 



General Electric Company, 15 watts, at 50 cm; 7 ergs/mm^/sec 

 Bacteria: E. coli B, 2 X 10« colls/ml = B 

 Platings: 0.05 ml of phage dilution plus 0.2 ml of B per plate 



The basic observation is illustrated in Table 9-2 and consists in the fact 

 that the same amount of irradiated phage gives a higher activity titer 

 (number of lytic areas or "placiues" on a solid layer of sensitive bacteria) 

 if the bacteria have been allowed to adsorb the phage from a more concen- 

 trated phage .suspension. The effect is not caused by exposure of the 

 infected bacteria to some factor other than phage present in crude concen- 

 trated phage preparations since it occurs equally well with purified phage. 



