402 



KADIATIUN UIOLOCJY 



Fig. V2-2, Init in tiilTereiit times, shorter times being required iit higher 

 intensities, within certain hmits (see also Seet. 3-5b). 



If the Hght intensity is very high, continuation of tiic illuininalion aftci- 

 the maximum is readied results in loss of infective centers; this elTect is 

 not specific for the photoreactivated particles, since it can be obtained 

 also when bacteria infected with active particles are exposed to a photo- 

 reactivating light of high intensity. Therefore a damaging effect of the 

 photoreactivating light on the phage-bacterium complex interferes with 

 the process of photoreactivation. This damage of the phage is not due to 



6 X 10^2 



10 20 30 40 50 60 70 



EXPOSURE TO REACTIVATING LIGHT, min 



Fk;. 12-2. Fraction of active particles in phaj^e sample as a function of the time of 

 illumination and of the light intensity. Phage T2 was irradiated for 20 sec with the 

 germicidal lamp, adsorbed on resting bacteria, and illuminated in liquid at 37°C. 

 Curve I was obtained with a light of intensity 10 (in arbitrary units), curve II with a 

 light of intensity 2.9, and curve III with a light of intensity O.G. {Dulhecco, 1950.) 



the light inactivation of the bacteria (IloUaender, 1943). This re(iuires 

 still higher doses (Dulbecco, unpublished). The damaging effect can be 

 reduced by reducing the intensity of the reactivating light ; this, as shown 

 later, affects photoreactivation only to a small extent, but decreases the 

 damaging effect. 



The logarithm of the fraction of photoreactivable particles that have 

 not been reactivated, log fl — p(/)/p(=o)], plotted versus the time of 

 illumination with a light of constant intensity is called the "photoreacti- 

 vation curve." In the case of phage T2 this curve is very nearly a 

 perfectly straight line (Fig. 12-8). 



Curves determined with the greatest accuracy show one and sometimes 

 two slight deviations from linearity. The first deviation always occurs at 

 very short periods of illumination, and is due to a relatively higher effi- 



