106 DISINFECTION 



Buchliolz and v. Jeney (1935) suggest that the reaction may possibly be of 

 photo-chemical nature. These workers point out that the highly lethal waves 

 2650 and 2530 A.U. correspond respectively to energy values of 4-6 and 4-8 

 volts, and conclude that this amount of energy is required to displace sufficient 

 electrons from the bacterial protoplasm to give rise to irreversible photo-chemical 

 alterations and thus bring about the death of the cell. Wyckoff (1932), who 

 has studied this aspect of the problem in the light of the quantum theory, finds 

 that about 4 milUon quanta of energy are required to kill a single coliform bacillus, 

 showing that death is not due, as it appears to be with the cathode rays, to a 

 single quantum absorption, but to some more generalized effect on the bacterial 

 protoplasm. According to Lea and Haines (1940), about 100 times more energy 

 is required to kill a bacterium when administered as ultra-violet light than as 

 X-rays, suggesting that the quantum yields of ultra-violet light are very small. 

 They conclude with Wyckoff that the energy of a single ultra-violet Ught quantum 

 is not usually sufficient to cause ionization. A similar conclusion has also been 

 reached by Rentschler, Nagy and Mouromseff (1941). 



The mode of action of ultra-violet light is still obscure, but the work of Gates 

 (1930) and Ehrismann and Noethling (1932) renders it probable that it depends 

 on the alteration of certain molecular groupings in the cell having high specific 

 absorption spectra for these rays. The exact nature of the effect produced must 

 await further observation. 



Photodynamic Sensitization. — We have seen that the visible rays of the spectrum 

 have only a weak germicidal action on bacteria. It has been found, however, by 

 Kaab (1900) and by v. Tappeiner (1900) that certain fluorescent dyes are able to 

 sensitize infusoria to the action of these rays, so that they become almost as lethal 

 as the ultra-violet rays. Thus it was shown that paramoecium suspended in a solu- 

 tion of acridin or eosin was killed very much more rapidly when exposed to diffuse 

 sunlight, which was itself harmless, than when kept in the dark. Examining this 

 phenomenon more closely, v. Tappeiner projected a spectrum across a table, and 

 placed a culture of paramoecium, suspended in 1-800 solution of eosin, in the red, 

 green, violet, and ultra-violet parts. The culture exposed to the green rays was 

 killed in 2 to 4 hours, whereas the cultures exposed to the other rays appeared 

 to be unharmed. This is of double interest ; firstly, because the green rays by 

 themselves are the least active in germicidal power ; and, secondly, because it is 

 in green light that eosin fluoresces most strongly. Similarly with acridin, death 

 was most rapid when the suspension was exposed to the violet rays, these being the 

 rays which cause acridin to fluoresce. For the sensitization to occur it was essential 

 for the paramcEcium to be in close contact with the fluorescing particles. When 

 the cultures were exposed to light that was simply filtered through eosin, they 

 remained unharmed ; the eosin had to be dissolved in the actual culture before 

 its sensitizing action became apparent. 



Later, v. Tappeiner and Jodlbauer (1904) and Jodlbauer and v, Tappeiner 

 (1904) demonstrated that the photodynamic action of dyes is manifest not only 

 in relation to infusoria, but in relation to bacteria, toxins, and to a less extent 

 antitoxins, while more recently the bacteriophage (Clifton 1931) and filtrable viruses 

 (Perdrau and Todd 19336) have been found susceptible. 



Burge and Neill (1915), exposing various micro-organisms to ultra-violet light, 

 found that the non-fluorescent were killed more rapidly than the fluorescent bac- 

 teria ; their supposition is that the latter protect themselves from the coagulating 



