EFFPX'TS OF RADIATION ON BACTEUIA 391 



Moisture Content. Somewhat conflicting results have been obtained by 

 different investigators who studied the relation between sensitivity to 

 ultraviolet and moisture content of the colls. Thus Roller (1939) and 

 Wells (1940) reported that air-borne K. coli are more sensitive than the 

 same organisms floating in liciuid suspension. Rentschler and Nagy 

 (1940) reported the same sensitivity for air-borne bacteria and for bacteria 

 exposed on the surface of agar plates. Wells and Wells (1936) and KoUer 

 (1939) observed that air-borne bacteria were more resistant to ultraviolet 

 irradiation at high than at low relative humidities. On the other hand, 

 Rentschler and Nagy (1940) found no difference in sensitivity of air-borne 

 bacteria at different relative humidities. A direct relation between X-ray 

 sensitivity of spores of A. terreus and their relative water content was 

 observed by Stapleton and Hollaender (1952). Spores containing approx- 

 imate!}'^ 25, 42, and 80 per cent moisture had relative sensitivities of 

 1 : 1.7:2.4, respectively. 



Enzymatic Constitution. The enzymatic constitution of bacterial cells 

 is known to depend on the conditions of growth. Thus the enzyme sys- 

 tems of resting bacteria differ from those of growing cells. Similarly, 

 resting cells grown in broth, glucose broth, or synthetic medium differ in 

 enzymatic constitution. Roberts and Aldous (1949) have shown corre- 

 sponding variation in the ultraviolet sensitivity of cells of E. coli B. 

 Resting cells grown in broth (final pH 8) were much less sensitive than 

 resting cells grown in glucose broth (final pH 5.5) or bacteria grown in 

 broth with a final pH of 7.0. Two-hour cultures of E. coli B grown in 

 broth and in synthetic medium were more sensitive than cultures of resting 

 cells grown in broth. Not only did the sensitivity change, but the form of 

 the survival curve changed with different growth conditions. In all cases, 

 greater survival was observed when the irradiated cells v.-ere assayed on 

 synthetic agar plates than when assayed on nutrient agar plates. Strain 

 B/r, the radiation-resistant mutant of strain B, did not yield similar 

 results. 



Genetic Constitution. Several references have already been made to 

 E. coli B/r isolated by Witkin (1946, 1947). This radiation-resistant 

 mutant was first isolated as one of four surviving colonies from a sample of 

 strain B, which had survived a dose of 1000 ergs/mm- of 2537 A ultraviolet 

 radiation. The mutant strain, which shows greatly increased resistance 

 to ultraviolet and ionizing radiations, has remained stable throughout 

 numerous transfers and has been widely used by many investigators in 

 radiobiological investigations. Witkin found that cells of the parent 

 strain, when subjected to low doses of ultraviolet and plated on agar, 

 formed almost 100 per cent of long filaments. Strain B/r, on the other 

 hand, after a normal lag period of about 1 hr, divided normally. Witkin 

 utilized this observation in a clever double-irradiation technique which 

 permits quantitative estimation of the numbers of resistant cells in 



