322 BACTERIOPHAGES 



of the mixed indicator method were confirmed by single burst 

 experiments. In mixed muhiple infections with Tl and T7 

 phages, simultaneous infection resulted in about Vs of the bac- 

 teria liberating only Tl and 2/3 only T7 phages. If either Tl 



Figure 10. Mutual exclusion between Tl and T7. The mixedly infected 

 bacteria were plated before lysis on the mixed indicator B/'l + B, 7. Note 

 that complete lysis occurs only where turbid plaques of the two kinds overlap. 



or T7 was given a 4 minute advantage, the other was com- 

 pletely excluded. For the three phages studied, the order of 

 dominance was T2 > T7 > Tl . 



Mixed multiple infection with two unrelated phages some- 

 times results in a markedly reduced yield of the phage that wins 

 out (Delbriick, 1945c). This phenomenon is termed "depressor 

 effect." The depressor effect depends on the times of infection 

 with the two phages. The longer the interval between addition 



