EFFECT OF ULTRAVIOLET RADIATION ON BACTERIA 



161 



Erythema 



In the destruction of yeast, Saccharomyces cerevisiae, the inhibitory 

 and lethal effects, according to Oster [1934], are approximately the same 

 for all wavelengths, while the destruction efficiency on the basis of a 

 50 per cent killing is a maximum at wavelengths between 2600 and 



o 



2700 A. The energy required to suppress budding of 50 per cent of the 

 cells irradiated ranges from 457 ergs/mm 2 at 2652 A to 23,500 ergs/mm 2 

 at 3022 A. Landen and Uber [1939] obtained 500 ergs/mm 2 as the 



o 



destructive efficiency of 2650 A for the yeast Sacc. ellipsoideus. Similar 

 results were obtained by Sharp [1938], for Bacillus anthraci using the 

 strong ultraviolet mercury line 2537 A. Summer sunlight cannot pro- 

 duce comparable effects since the normal atmosphere absorbs all lethal 

 radiation shorter than 2950 A. 



Gates [1929] concludes from his study of the bactericidal action of 

 ultraviolet light on S. aureus that : (1) in the initial period of exposure no 

 bacteria succumb; (2) after this initial exposure a considerable number 

 of bacteria, between 20 and 30 per cent, are destroyed, and in this group 

 are found the young ones ; (3) the 

 remainder to about 70 or 80 per 

 cent of the total number succumb 

 along an energy gradient that ap- 

 pears to have an exponential rela- 

 tionship to the lethal effects; and 

 finally (4) a number of organisms 

 remain which require an excess of 

 energy to kill them. 



The evidence, therefore, shows 

 that the most effective bacterici- 

 dal region lies in the range 2500 



o 



to 2650 A, with a possible maxi- 



o ^^ 



mum effectiveness at 2650 A. The 

 absorption maxima for proteins, 

 yeast, and pepsin also extend from 

 2700 to 2800 A, from which it may 

 be inferred that bacteria are de- 

 stroyed by the photochemical ioni- 

 zation induced in the protein body 

 material of the bacterium (Fig. 

 IV-19), if the |surface of the bacte- 

 rium and the medium in which it is 

 investigated are excluded. Since 

 at 2700 Ait takes about 25 X 10~ 6 

 erg per bacterium (Hercik [1936]) 



to _ 



o 



-a 

 < - 



2400 



2600 2800 



Wavelength 



3000 



3200 A 



Fig. IV-19. The antirachitic response 

 for an equal energy spectrum is shown as 

 compared with the spectral absorption of 

 ergosterol and the average erythemic re- 

 action of the untanned skin to an equal 

 energy spectrum. 



Antirachitic data from Knudson and 

 Benford [1938]. 



Ergosterol data from Bills, Honeywell, 

 and MacNair [1928]. 



Erythema data from Luckiesh, Holla- 

 day, and Taylor [1930] or Coblentz, 

 Starr, and Hogue [1932]. 



