5 1 4 BAf :tf.riophages 



visible light is administered during a limited period after the in- 

 activated phage particles have been adsorbed to host bacteria. 

 Illumination of free virus particles or uninfected bacteria has no 

 reactivating effect on inactivated phage subsequently adsorbed. 

 For illumination on the plate, Dulbecco used two parallel fluo- 

 rescent discharge lamps (40 watts each) at a distance of 12 in. 

 from the plate. For illumination of liquid media, a General Elec- 

 tric H-5 light source was used together with one or more filters. 

 Dulbecco (1950) has given a complete description of the appara- 

 tus used for illumination in liquid media.] 



Multiplicity Reactivation of Ultraviolet-Inactivated Phages 



Luria (1947) found that bacteria singly infected with inacti- 

 vated phage produced no plaques when plated. However, any 

 bacterium infected with 2 or more inactive phage particles did 

 produce plaques if not too many r doses of irradiation had been 

 given. 



The average multiplicity of infection, n, with ultraviolet-inac- 

 tivated phage particles can be obtained as described above by 

 determining the proportion of surviving bacteria at the end of 

 the adsorption period. The proportion of uninfected bacteria is 

 related to n by the Poisson formula P{0) = e~". Once n has been 

 determined in this way, the proportion of singly infected bacteria 

 can be calculated from the formula P{\) = n 6-"". The propor- 

 tion of bacteria which had adsorbed 2 or more phage particles is 

 then 1 - {P(0) + P(l)} = 1 - (« + 1) e-", and the actual 

 number of bacteria which have been multiply infected is equal 

 to 1 — (n + 1) e~" times the total number of bacteria. The 

 number of bacteria which have the ability to produce plaques 

 can be determined by the plating of appropriate dilutions by the 

 agar layer method and counting the plaques, correcting the 

 count if necessary for non-inactivated phage particles. 



Example: In an actual experiment a suspension of 10^ 5/ml. was mixed 

 with various amounts of an irradiated T4 phage stock. The adsorption mix- 



