324 BACTERIOPHAGES 



lambda does not interfere with the multiphcation of phage T5. 

 Treatment of K12 with m.ild doses of uhraviolet Hght initiates 

 the lytic cycle of growth of phage lambda resulting in lysis some 

 60 to 80 minutes after irradiation. Mature phage particles 

 first appear in the bacteria 40 to 45 minutes after irradiation. 



Suspensions of K12 were irradiated, infected with phage T5 

 at various intervals after irradiation, and then assayed on 

 appropriate indicators to determine the proportion of cells liber- 

 ating phage T5 or phage lambda. The proportion of bacteria 

 liberating lambda was less than one per cent at all intervals up to 

 20 minutes. It reached about 10 per cent after a 37-minute 

 interval and about 80 per cent after a 50-minute interval between 

 irradiation and superinfection. The proportion of bacteria 

 liberating phage T5 was as high as in the nonirradiated control 

 at all intervals up to 20 minutes and then decreased at longer 

 intervals in the same way as the proportion of lambda-yielding 

 bacteria increased. Platings on mixed indicator strains showed 

 that the proportion of bacteria liberating both T5 and lambda 

 was negligible, so that this is a typical example of mutual ex- 

 clusion. The invading T5 phage is dominant at all times after 

 irradiation until the maturation of lambda begins. After this 

 time lambda is dominant over phage T5. There was no evi- 

 dence for a depressor effect in these experiments. 



It is obvious that in this case the penetration hypothesis 

 cannot be invoked as an explanation of mutual exclusion because 

 the lambda prophage excluded by T5 was already present within 

 the bacteria. 



Similar results have been obtained with the unrelated tem- 

 perate phages PI and P8 of Pseudomonas aeruginosa. In mixed 

 infections of the sensitive host strain 13 with PI and P8, mutual 

 exclusion occurs and the majority of the mixedly infected bacteria 

 liberate PI. However, lysogenic strain 13(8) is a satisfactory 

 host for PI and lysogenic host 13(1) supports growth of P8. 

 Therefore mutual exclusion operates in this system only during 

 the lytic cycle of phage growth (Jacob, 1 954a). If the lysogenic 

 strain 13(8) is induced with ultraviolet light and then super- 



