MIXED INFECTION 321 



Tl. Phage Tl is effectively excluded from multiplying in bac- 

 teria that are infected with phage T2. The Tl phage particles 

 that are adsorbed to T2-infected bacteria are inactivated; 

 they are not recoverable as plaque-forming phage either before 

 or after lysis of the mixedly infected bacteria. In the light 

 of contemporary knowledge, we suppose that both phages inject, 

 but only T2 carries through all the steps necessary to production 

 of new phage particles. 



Phage Tl was given a head start of various time intervals 

 before addition of phage T2 to see if it could escape domination 

 by this means. When Tl preceded T2 by 2 or 4 minutes, T2 

 still dominated growth. When Tl preceded T2 by 6.5 minutes, 

 it accounted for about 60 per cent of the yield. If Tl preceded 

 by 7.5 minutes, it made up nearly 90 per cent of the yield. 

 Tl must precede T2 by about 6 minutes in order to compete on 

 an equal basis with T2 in mixed infection. 



In a later study of mixed infection, Delbriick (1945c) intro- 

 duced a new and valuable technique, the use of mixed indicator 

 bacteria as a test for the simultaneous liberation of different 

 phages from the same bacterium. To detect mixed yields 

 of Tl and T2, for instance, he seeded plates with a mixture of 

 B/1 and B/2, together with about 100 of the mixedly infected 

 bacteria under test. If, on such a plate, an infected bacterium 

 liberates only phage T2, the indicator B/1 is lysed producing a 

 plaque that is turbid due to growth of B/2. Similarly, a bac- 

 terium liberating only phage Tl produces a turbid plaque be- 

 cause B/2 is lysed but not B/1. If plaques of Tl and T2 over- 

 lap on such a plate, the overlap is completely clear. If a single 

 bacterium liberates both phages, the result is a concentric over- 

 lap, the clear center containing both phages, the turbid periphery 

 Tl only. This method permits the detection of mixed yielders 

 in any proportion in excess of about one per cent of the popula- 

 tion. An example of mutual exclusion revealed in this way is 

 illustrated in Figure 10. 



In applying this technique to the phage pairs T1-T7 and 

 T2-T7, Delbriick could not detect mixed yielders. The results 



