64 BACTERIOPHAGES 



radiations is in the vicinity of 4,500 A, or in the blue region. 

 He also observed that, like SI 3, a large subtilis phage is sensi- 

 tive, while the phages coli 36 and streptococcus B563, like CI 6, 

 are resistant to inactivation by visible light. This differential 

 action of visible radiations is presumably due to some difference in 

 the chemical composition of these phage strains, in that only the 

 light-sensitive phage types contain some pigment which absorbs 

 blue light. Wahl and Latarjet (1947) examined the inacti- 

 vation spectrum of SI 3 more critically and found that light of 

 wavelengths greater than 5,550 A is ineffective, the efficiency of 

 inactivation increasing with decreasing wavelength down through 

 3,650 A. Throughout this part of the spectrum phage SI 3 is 

 more radiosensitive than phage CI 6. Between 3,130 and 

 3,650 A, an inversion of the relative sensitivities of these two 

 phage strains occurs, so that at shorter wave lengths in the ul- 

 traviolet CI 6 becomes more radiosensitive than SI 3. The ki- 

 netics of inactivation of SI 3 by light at both 3,650 and 4,500 A 

 are complex in that the first 75 per cent of the phage particles 

 are killed according to an exponential survival curve (see 

 Glossary) of greater slope than the rate of inactivation of the 

 remaining 25 per cent. A change of temperature from 17 

 to 37° C. does not change the rate of inactivation of phage 

 SI 3 at 4,500 A, indicating that the lethal action results from a 

 direct photochemical effect. The effectiveness of the incident 

 energy in the visible region at 4,500 A is only 10~'* of that in the 

 ultraviolet region at 2,537 A, although no data are available for 

 such a comparison in terms of the absorbed energy. The dif- 

 ferential sensitivity of phages to visible light may be an im- 

 portant clue to their chemJcal structure, although no informa- 

 tion appears to be as yet at hand concerning the nature of the 

 material which absorbs the blue light. 



The photodynamic action of dyes on a bacteriophage was inves- 

 tigated by Clifton (1931) who observed that addition of 0.01 per 

 cent methylene blue to a suspension of a staphylococcus phage 

 has no effect on the viability of the phage particles as long as the 

 suspension is kept in the dark but results in complete inactivation 



