EFFECTS OF RADIATION ON BACTERIA 389 



RELATIVE SENSITIVITY OF VARIOUS SPECIES 



Comparison of the sensitivity of different species in absolute energy- 

 units must be made with caution since there are many factors which can 

 influence the results obtained by different investigators in different labo- 

 ratories. In Table 10-4 are shown the incident energies of 2537 A ultra- 

 violet in ergs per square centimeter necessary to inhibit colony formation 

 in 90 per cent of the organisms. This table is taken from Hollaender 

 (1942) and includes some estimates of the sensitivity of E. coli B and B/r 

 made from the data of Demerec and Latarjet (1946) and Witkin (1947). 

 These estimates show a striking difference between the two strains of 

 bacteria, one of which is a radiation-resistant mutant of the other. It is 

 of interest that E. coli B is the most sensitive strain for which data are 

 available. 



FACTORS INFLUENCING SENSITIVITY 



pH. Bay ne- Jones and Van der Lingen (1923) and Gates (1929b) 

 \'aried the pH of the medium on which the organisms were irradiated 

 between 4.5 and 9. No appreciable influence of pH on the bactericidal 

 effect was observed although the former workers observed more rapid 

 inactivation below pH 4.6. 



Stage of Growth. Relatively few investigations have been concerned 

 with the comparative sensitivity of bacteria to ultraviolet at different 

 stages in the growth cycle. Morse and Carter (1949) and Morse (1950) 

 have shown about threefold variation in the DNA content of cells of E. 

 coli B and B/r at different stages of the growth cycle. In view of the 

 maximum at 2600 A in the bactericidal action spectra, it would be sur- 

 prising if the sensiti\'ity of bacteria remained constant during the growth 

 cycle. Hollaender and Claus (1936) found 7-hr agar slant cultures of 

 E. coli to be more resistant to ultraviolet than their standard 15-hr cul- 

 tures, whereas 10-day-old cultures were more sensitive. Microscopic 

 examination of the 7-hr cultures revealed that 70 per cent of the cells were 

 double, which may account for a large part of the difference in resistance 

 observed. Demerec and Latarjet (1946) observed that growing cells of E. 

 coli B/r were more sensitive to 2537 A ultraviolet than were resting cells. 

 Witkin (1951) reported greatest sensitivity to ultraviolet in the loga- 

 rithmic phase, greater resistance in the resting stage, and the greatest 

 resistance during the lag phase. These differences in resistance did not 

 parallel differences observed in the mean number of nuclei per cell. 



Relative Sensitivity of Vegetative Cells and Spores. Few investigators 

 have compared the sensitivity of spores with that of vegetative cells of the 

 same strain. Duggar and Hollaender (1934b) observed B. subtilis spores 

 to be about twice as resistant as the vegetative cells. This was confirmed 

 by Hercik (1937) with B. megatherium and l)y Rentschler et al. (1941) 

 with B. .mbtilis (Table 10-4). 



