EFEFCTS OF RADIATION ON BACTERIA 397 



this wave length sensitized bacterial spores to subseciuent exposure to heat. 

 Schumann rays of 1250-1 GOO A were several times as effective as 2537 A 

 in producing this heat sensitization. 



NEAR-ULTRAVIOLET AND SHORT-VISIBLE RADIATION 



Somewhat conflicting results have been obtained by different investi- 

 gators in studies of the bactericidal action of the near ultraviolet. The 

 upper limit of the bactericidally effective wave lengths as reported by dif- 

 ferent investigators has ranged from 2967 to as high as 3650 A. However, 

 there seems little doubt that, although the efficiency of the w^ave lengths 

 above about 2967 A decreases greatly corresponding to a similar low 

 absorption of these wave lengths by bacterial cells, exposure to large 

 amounts of radiation in the near-ultraviolet and short-visible regions will 

 produce bactericidal effects as measured by viable count. Duggar (1936) 

 reviews many of the earlier data. The most extensive data are those of 

 Hollaender (1943) who showed a significant reduction in viable count 

 following large exposures to radiation in the region from 3500 to 4900 A 

 but greatest near 3650 A. Typical survival curves for the same culture of 

 E. coli exposed to 2650 A and 3500-4900 A radiation are shown in Fig. 

 10-10. The survival curve obtained with the latter range of radiation is 

 definitely of a threshold type, no bactericidal effects being noticed until 

 large amounts of energy have been absorbed. Several differences were 

 noticed in the studies of near-ultraviolet and short-visible radiation as 

 compared to the bactericidally effective wave lengths. A temperature 

 coefficient of about 1.7-2.2 for the near ultraviolet contrasts with the 

 value of 1.1 for ultraviolet (Gates, 1929b). The incident energy for a 

 given lethal effect is much greater for the near ultraviolet, and the exten- 

 sion of the lag phase is more pronounced. Reference has already been 

 made to the observation that cells exposed to near ultraviolet are more 

 sensitive to the toxic effect of suspension in saline at 37°C. In Table 10-5 

 the differences between near-ultraviolet and the bactericidally effective 

 wave lengths are summarized. Hollaender postulates the destruction of 

 an essential cell component, or components, probably generally distrib- 

 uted throughout the cell, which cannot be repaired or replaced from the 

 other cell constituents but can, in many of the cells, be replaced from 

 factors which are present in nutrient broth as the mechanism for the 

 lethal action of near ultraviolet. 



The effect of treatment with near-ultraviolet and short-visible radiation 

 in causing photoreversal of ultraviolet effects has already been discussed 

 briefly. In this connection, Heinmets and Taylor (1951) have studied 

 effects of ultraviolet and near-ultraviolet radiation and visible light on 

 frozen bacteria. Photoreactivation was observed to occur only in the 

 li(iuid phase. Bacteria in the frozen state, whether or not previousl^y 

 exposed to ultraviolet, were inactivated by radiation in the 3000-4000 A 



