APPENDIX 517 



determined at the end of the experiment. The bacteria in the 

 virus-free control took up oxygen at a rate which increased ex- 

 ponentially with time in the same way that the bacterial popula- 

 tion increased. The bacteria infected with either active T2 

 or ultraviolet-inactivated T2 continued to take up oxygen at 

 the rate which prevailed at the moment of infection. There 

 was no increase in oxygen absorption rate or in bacterial tur- 

 bidity in the vessels containing phage. The respiratory quo- 

 tient was the same in all 3 vessels, about 1.05. This experiment 

 indicates that both active and ultraviolet-inactivated T2 phage 

 can interrupt the multiplication of infected host cells without 

 altering the rate of oxygen uptake or CO2 evolution. Infection 

 with phage does not affect host cell respiration. Use of T2r+ 

 phage in these experiments resulted in the phenomenon of lysis 

 inhibition (p. 506) and enabled continuation of experiments for 

 several hours before lysis began to interfere with the observa- 

 tions. 



Cohen (1948) continued his study of virus-infected host cells 

 by making chemical analyses for protein and nucleic acid at 

 intervals during the latent period. This was done by adding 

 trichloroacetic acid to aliquots of the culture to a final concen- 

 tration of 5 per cent. The mixture was chilled 15 min. and 

 centrifuged 10 min. The supernatant fluid was decanted, 

 the tubes were drained and the precipitate washed with cold 5 

 per cent trichloroacetic acid. The precipitate could then be 

 analyzed for total phosphorus, for nitrogen by Kjeldahl, for 

 pentose by quantitative Bial (Mejbaum, 1939) or for desoxyri- 

 bosenucleic acid (DNA) by the diphenylamine reaction (Dische, 

 1930). About 10^ bacteria are required for these analyses 

 except for Kjeldahl N, for which about 10^° bacteria are needed. 



Another method of following intracellular phage synthesis 

 was developed by Racker and Adams (unpublished experi- 

 ments) as a result of Cohen's work. Nucleic acids have a high 

 absorption of ultraviolet radiation at 2,600 A. Since phage 



