APPENDIX 499 



1 . Mixed infection of strain B with Tl and T7 : Mutual exclu- 

 sion effect. The mixed infection experiment was performed to 

 see what would happen when a bacterial cell was simultaneously 

 infected with 2 difTerent viruses (Delbriick, 1945c). The pro- 

 cedure involved adding a mixture of the 2 viruses to a suspension 

 of bacteria at an input ratio such that after adsorption the aver- 

 age multiplicity of infection would be about 3 for each phage 

 type. At least 90 per cent of the bacteria would then have ad- 

 sorbed at least 1 phage particle of each kind and hence would be 

 mixedly infected (p. 479). At the end of the adsorption period 

 the adsorption mixture was diluted into a mixture of the 2 anti- 

 sera to stop adsorption and to neutralize unadsorbed phage. 

 After further dilution into F.G.T. the infected bacteria were 

 plated by the agar layer method on strain B, on the separate in- 

 dicator strains and on mixed indicator strains before the end of 

 the latent period. Also, plates were made from S.G.T. after the 

 end of the rise period on the separate indicator strains for deter- 

 mination of burst size. 



The number of infected bacteria yielding the 2 types of phage 

 are determined from the plaque counts before the end of the 

 latent period. Infected bacteria were plated with the following 

 results : 



on B 101 plaques (total of Tl and T7 yielders) 



on B/1,5 35 plaques (T7 yielders) 



on B/3,4,7 70 plaques (Tl yielders) 



on mixed indicators 92 turbid plaques (bacteria yielding either Tl 

 orT7) 

 3 clear plaques (bacteria yielding both Tl and 

 T7) 



The sum of the bacteria which liberate Tl and those which lib- 

 erate T7 equals the total number of infected bacteria when as- 

 sayed on B, and also is equal to the assay of the bacteria put into 

 the adsorption tube since at a multiplicity of 3 nearly all bacteria 

 are infected. This result indicates that an infected bacterium 

 liberates either Tl or T7 but not both, within the sampling er- 



