332 



R. DULBECCO 



[vol. 59 



Illumination of bacteria alone followed by the addition of UVP does not pro- 

 duce any PHTR. Bacteria spread on the surface of several nutrient-agar plates 

 were exposed to the light of a fluorescent lamp (80 watts, 12-inch distance) for 

 4 hours at room temperature ; then UVP were spread on the same plates in dark- 

 ness, and the plates were incubated in darkness. Control plates, spread with 

 bacteria at the same time, were kept in darkness and received UVP at the same 

 time as the illuminated plates. Equal numbers of plaques were found in all plates 

 whether the bacteria had been preilluminated or not, showing that preillumina- 

 tion of bacteria does not cause PHTR of UVP added later. In another experi- 

 ment a suspension of resting bacteria was illuminated with a light of 365-m/i 

 wave length at 37 C for a period long enough to give a very high PHTR in ad- 



TABLE 1 



Ejfect of light on inactivated phage T2 alone and on unadsorhed inactivated phage T2r mixed 



with sensitive bacteria 



sorbed UVP, and the UVP was added at the very moment at which the light 

 was turned off; no measurable PHTR was observed. 



If bacteria killed by heating to 60 C for 20 minutes are substituted for living 

 bacteria, no PHTR takes place. Actually the plaque count decreases, probably 

 because of an irreversible adsorption of phage by the dead bacteria without the 

 release of new phage. 



Illumination of bacteria prior to infection does not diminish the photoreac- 

 tivability of UVP added later, as shown by the following experiment: Bacteria 

 were spread on the surface of nutrient agar plates and exposed to the light of a 

 fluorescent lamp (80 watts, 12-inch distance) for 4 hours at room temperature; 

 then UVP was spread on the plates, which were afterwards incubated under the 

 same light. After incubation the plates showed the same number of plaques as 

 control plates containing nonpreilluminated bacteria and UVP, incubated under 

 the same light. 



231 



