66 BACTERIOPHAGES 



activating phage particles, ultraviolet light produces important 

 physiological and genetic effects, such as inducing phage de- 

 velopment in lysogenic bacteria, causing a growth-delay in the 

 phage particles surviving irradiation, stimulating genetic re- 

 combination, and exerting a mutagenic action. Some of the 

 effects of ultraviolet light, furthermore, are reversible under ap- 

 propriate conditions. Ultraviolet light has also been used to 

 study the course of intracellular phage development. The pos- 

 sibilities of ultraviolet light as a tool in phage research appear to 

 be far from exhausted. 



a. Lethal Effect 



Repeated observations of the lethal effect of ultraviolet light 

 on bacteriophages have been made ever since 1922 (see d'- 

 Herelle, 1926). The first quantitative study appears to have 

 been carried out by Baker and Nanavutty (1929). These authors 

 found that the inactivation of a shiga phage is an exponential 

 function of the dose of ultraviolet light to a survival of 10"\ 

 By use of filters it was possible to determine that wavelengths 

 above 3,000 A are harmless and that the wavelength of maximum 

 effectiveness is somewhere below 2,650 A. Gates (1934), using 

 a monochromator to isolate single emission lines, studied the 

 inactivation of a staphylococcus phage and found that the 

 kinetics of inactivation follow a simple exponential at all wave- 

 lengths studied from 2,300 to 2,970 A. The energy output of the 

 radiation source was determined at each wave length by means 

 of a thermopile, thus permitting a determination of the action 

 spectrum for the ultraviolet inactivation of this phage. It was 

 found that the incident energy required to inactivate a given 

 fraction of the phage population decreases continuously between 

 2,950 and 2,600 A, then increases to a peak at 2,400 A and finally 

 decreases again toward 2,300 A. Northrop (1938) determined 

 the ultraviolet absorption spectrum of purified staphylococcus 

 phage and found it to be very similar in shape to Gates' action 

 spectrum. Analogous investigations carried out subsequently to 

 determine the action spectra of ultraviolet inactivation of Tl, T2, 



