94 S. E. LURIA AND R. DULBECCO 



bacterial mutants resistant to one or more phages as indicators for one phage 

 in the presence of another (see Delbruck 1946). Plate counts for viable bac- 

 teria, and plaque counts in agar layer for active phage were used throughout, 

 employing 1.1 percent agar in "Difco" nutrient broth plus 0.5 percent NaCl. 

 All plates were incubated at 37°C. Experimental bacterial cultures in the 

 logarithmic phase of growth were grown with aeration at 37°C from standard 

 inocula. 



The phage stocks were lysates in glucose+ammonia (or lactate+ammonia) 

 medium. These media give negligible absorption of the ultraviolet light used 

 in this work. High titer phage lysates (over 1X10" particles per ml) might 

 give some ultraviolet screening effect because of bacterial debris and of phage 

 itself. Whenever possible, therefore, phage was irradiated after a dilution 1 :5 

 or higher in the same medium. The source of ultraviolet was a General Electric 

 Company germicidal bulb, 15 watts, alimented through a stabilizer. At a 

 distance of 50 cm from the center of this bulb, the flux— measured with a West- 

 inghouse SM-200 meter with tantalum phototube WL-775 — is about 7 

 ergXmm"^ sec~^ The beam contains mainly radiation of wavelength 2537 A. 

 Samples were irradiated in a thin layer (not over 0.4 mm) in open Petri 

 dishes rocked during exposure. 



The technique of "one-step growth" experiment in its various forms has 

 been described in detail previously (Delbruck and Luria 1942). 



EXPERIMENTAL 



Inactivalion and reactivation of bacteriophages 



Plaque counts on phage suspensions exposed to ultraviolet for various lengths 

 of time generally give survival ratios whose logarithms are proportional to the 

 dose, that is, to the time of exposure (see Latarjet and Wahl 1945, and 

 figure 1). The logarithmic rate indicates a one-hit mechanism of inactivation 

 (Lea 1947), and we can assume that the hit consists of the successful absorption 

 of one quantum. The probability that one quantum produces inactivation is, 

 however, very small: for phage T2, for example, one inactivating hit is pro- 

 duced by a dose corresponding to almost 10^ quanta absorbed per particle 

 (M. Zelle, personal communication). Only one absorption in 10^ on the aver- 

 age is, therefore, effective, the others probably producing excitations that do 

 not lead to the inactivating effect. 



When the average number of effective hits per particle is r, the proportion 

 of active to total phage will be e"*". For r = 1, e~'' = 0.37 ; the corresponding dose 

 is the "inactivation dose" in Lea's terminology (1947). If doses are expressed 

 in multiples of the inactivation dose, their values give directly the average 

 number of hits per particle. 



Phage particles inactivated by ultraviolet light are adsorbed by bacteria 

 (Luria and Delbruck 1942). This is detected because adsorption of one par- 

 ticle by a bacterium causes death of the latter. One can, therefore, measure 

 the rate of adsorption of inactive particles from the survival of bacteria in 

 mixtures containing bacteria and irradiated phage in known proportions. 

 If, on the average, x particles are adsorbed per bacterium, a fraction e~' of the 



248 



