The Biochemistry of Lysogeny 



43 



As the bacterial population reaches a stationary level upon the ex- 

 haustion of the carbon source, the accumulation of ultraviolet-absorb- 

 ing substances also diminishes. This again is not a characteristic of 

 lysogenic bacteria alone, for E. coli B under similar conditions of cul- 

 ture repeats this pattern. 



That the excretion of ultraviolet-absorbing material per bacterial 

 cell in a culture in logarithmic growth phase is quite constant is appar- 



Low glucose (0.05J&) 

 9.20 



3.40 



12 3 4 5 6 7 



Hours 



Fig. 4. Same data as Fig. 3 but plotting the log of O.D. 



ent from Fig. 4. The plot of the log of the increasing bacterial popula- 

 tion and of the log of the optical density of the culture medium are 

 parallel. The constancy of the excretion was also shown by a different 

 method. Cultures of E. coli K i2 were kept in extended logarithmic 

 growth phase at a concentration of 10 8 cells per milliliter by the con- 

 stant dilution of the culture with fresh medium at 37° at a rate 

 which doubled the volume of the culture per generation time. Cell 

 counts and the optical density of the cell-free supernatant fluid were 

 determined for several hours. In such experiments, the optical density 

 at 260 m/x remained 0.07 with 10 8 cells per milliliter. 



The nature of the substances which account for the ultraviolet 

 absorption in the culture media of bacteria in logarithmic growth phase 

 on low glucose was investigated. Paper chromatography of lyophilized 

 concentrates revealed the presence of nucleic acid fragments and of 



