1950] PHOTOREACTIVATION OF INACTIVATED BACTERIOPHAGES 335 



This decrease in PHTR might be caused by a gradual decrease in the amount 

 of PHTR per time unit as the time interval between infection and illumination 

 increases, by a limitation of the time interval after infection in which PHTR 

 can occur, or by both. The amount of PHTR per time unit was determined in 

 experiments in which exposure to light was started at various times after in- 

 fection. The results, shown in figure 2, indicate that the amount of PHTR per 

 time unit remained practically constant for about 15 minutes. The decline in 

 maximum PHTR must be due, therefore, to a limitation of the time within which 

 PHTR can occur after infection, the useful time interval ending between 20 and 

 30 minutes after infection under the experimental conditions; after this time very 

 little or no PHTR can take place. 



Experiments with resting bacteria. As is shown in table 4, the maximum amount 

 of PHTR obtainable in phage T2r irradiated with the germicidal lamp for 18 

 seconds remains fairly constant for at least 70 minutes after infection at 37 C; 



TABLE 3 

 The effect of the time interval between infection and exposure to light (bacteria in broth) 

 Phage T2r, irradiated for 20 seconds with the germicidal lamp, was mixed with bacteria 

 and adsorption was allowed to continued for 2 minutes, after which it was interrupted by 

 serum anti-T2. Exposure to light (H-4 lamp, 12-inch distance) was begun at various times 

 and was continued for 100 minutes at 28 C. Amount of PHTR is lower than in experiment 

 reported in table 4, because in the present experiment a lower light intensity was used, 

 and the time in which the light could be utilized for reactivation was limited, since bacteria 

 in broth were used. 



longer intervals have not been tested. The amount of PHTR per time unit is 

 not influenced by the time interval between infection and illumination. 



The differences between experiments with bacteria in broth and with resting 

 bacteria indicate that under the experimental conditions the system "UVP- 

 metabolizing bacteria" undergoes a gradual change that in its late phases pre- 

 vents PHTR, a change absent in the system "UVP-resting bacteria." 



Kinetics of PHTR 



PHTR as a function of the time of exposure to the reactivating light. The following 

 experiments employed inactive phage T2r and resting bacteria, with illumina- 

 tion in liquid. Inactive phage diluted in phosphate buffer was mixed with bac- 

 teria at time at 37 C in darkness, and 10 minutes were allowed for complete 

 adsorption. At the eleventh minute a sample was plated in darkness; at the 

 twelfth minute the mixture was exposed to light, and samples were taken at 



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