EFFECTS OF RADIATION ON BACTERIA 1129 



order to tost the effect upon reaction velocity, suspensions of the bacteria 

 {Staphylococcus aureus) were prepared in phosphate buffers over the range 

 pH 2 to 9. Drops of the suspension on cover glasses were exposed at 

 25°C. to the zinc spark for measured periods of time and subsequently- 

 transferred to nutrient broth. Very slight change occurred in the time 

 required to kill between pH 4.6 and 9. Below pH 4.6, the isoelectric 

 point of the bacteria, the time to kill drops off suddenly, so that accelera- 

 tion of the killing action is marked in the more acid media. This is 

 interpreted to indicate that "the nature of the electric charge upon a 

 living organism radiated by ultra-violet has a direct influence upon the 

 destructive action of that radiation." 



Gates (59), working also with Staphylococcus aureus, made determina- 

 tions only within the range pH 4.5 and 7.5, and found no appreciable 

 influence upon the bactericidal reaction. 



Effects on the Medium. — Seeking an explanation of the lethal effect 

 of light on organisms, many of the early workers were led by several 

 circumstances, not primarily experimental, to attribute these effects 

 to the result of oxidation. In the first place, there was current the view 

 that, in general, the destruction of organic material results primarily 

 through oxidative processes. It was also a relatively simple explanation 

 to consider that the effect was indirect, induced through the action of 

 light on the substrate or upon the water of the substrate. The view that 

 oxygen is chiefly important in the killing effect seemed to be confirmed 

 by the fact that stirring or shaking exposed fluids enhanced killing 

 efficiency, whereas stirring or shaking in the case of exposed fluids is, 

 of course, essential in order that all organisms may be brought more or 

 less equally into the path of the beam of light. Again, the observation 

 was made that organisms at the surface of a liquid were more readily 

 killed, and this was thought to be connected with the oxygen supply. 

 The low penetrability, at least of the ultra-violet, was not always recog- 

 nized. However, a certain number of observations by early workers such 

 as Ward (155), D'Arcy and Hardy (3), Dieudomie (32), and others 

 seemed to confirm the general view that oxidation is important. On 

 the other hand, experimental evidence against the view promptly accumu- 

 lated. In particular, Bie (16) showed that strong bactericidal action of 

 ultra-violet could be demonstrated in the absence of any free oxygen. 

 Henri (73a) was definite, perhaps largely on theoretical grounds, in 

 declaring that the effect of light is direct, that is, on the protoplasm and 

 not through the formation of peroxide. 



Thiele and Wolfe (148) gave special attention to this problem. They 

 purified the gases bubbled through an exposure vessel until no trace of 

 oxygen could be detected, so that they came to the conclusion that light 

 action is wholly independent of the presence of oxygen. Likewise they 

 determined that peroxide has no indirect influence on the process. 



