384 



UADIATION IJIOLUUV 



vatetl and that wave k'lijjjtlis i)el()\v alxjut 31)50 A were hactericidally effec- 

 tive. Numerous workers had shown that the bactericidal eifectiveness 

 increased greatly for those wave lengths helow about 29()7 A and extended 

 to the siiortest wave lengtlis convenienlly studied. Furthermore, several 

 early investigators had suggested that absorption of ultraviolet radiation 

 by specific structures within I he ceil was responsible for the bactericidal 

 effects. For example, Henri (1914) emphasized absorption within the 

 nucleus as being primarily responsible for the inactivation and pointed out 

 the possibility, later realized in experiment, that sublethal doses might 

 induce hci'ilable modifications. 



I- 



8- 



V 



1^ 



E 



ACTION SPECTRA 



In 1928 Gates made the preliminary aiuiouiiccmcnt of the action spec- 

 trum or relative bactericidal effectiveness of diffei-cnt wave lengths of 

 monochromatic ultraviolet radiation and pointed out the probable rela- 

 tion to the absorption of ultraviolet by deoxyribonucleic acid (DNA) 



derivatives. His detailed results 



were presented in a later series of 



./• 'X publications (Gates, 1929a, b, 1930). 



Cells of S. aureus were irradiated on 

 the surface of agar plates with beams 

 of monochromatic ultraviolet radia- 

 tion isolated by means of a large 

 monochromator with quartz prisms, 

 the incident energy being measured 

 by means of a calibrated thermopile. 

 The studies were later extended to 

 E. coll (Gates, 1930), and measure- 

 ments were made of the absorption 

 coefficients of a thin layer of bac- 

 terial cells pressed between quartz 

 cover slips. For both species the 

 curves of the reciprocal of the inci- 

 dent energy required for 50 per cent 

 killing plotted against wave length 

 were similar to the absorption spec- 

 tra with a maximum of bactericidal 



I 



8 



6- 



O 4 



23 



25 27 



WAVE LENGTHS,/! 



— I — 



029 



31 



Fui. 10-7. Curve of the reciprocals of 

 the incident energies required for inacti- 

 vation of 50 per cent of E. coli. 

 (Adapted from Galen, 19:i0.) 



effectiveness and absorption of about 2000 A and a minimum at about 

 2380 A with indications of another maximum at wave lengths shorter 

 than 2300 A. The action spectrum for E. coli is reproduced in Fig. 

 10-7. 



Similar bactericidal action spectra have been observed by other workers 

 (Ehrismann and Noethling, 1932; Wyckoff, 1932; Duggar and HoUaender. 

 1934a, b; Ilollaender and Glaus, 1936; HoUaender and Duggar, 193(); 



