430 BACTERIOPHAGES 



the bacterial cells are nearly simultaneously infected with at least 

 one phage particle of each kind. 



Such a mixed infection experiment may have sev^eral possible 

 results: 



7. Mutual Exclusion in which a mixedly infected cell liberates 

 one or the other of the infecting phage types but never both 

 (Delbriick and Luria, 1 942) . This result has been obtained with 

 1 5 pairs of unrelated phage strains in the T-system of coliphages 

 and may be accepted as the usual result of mixed infection with 

 unrelated virulent phages. 



2. Mixed Growth without Genetic Recombination. In this case 

 mixedly infected bacteria liberate both kinds of phage but no 

 recombinants are found. This result has been found in one case 

 of mixed infection with unrelated phages studied by Collins 

 (1957) . How rare or common this result may be we cannot antic- 

 ipate at present. Many more systems must be studied before 

 the significance of such a result is understocd. 



3. Mixed Growth with Genetic Recombination. In this type of re- 

 sult the phage progeny from mixedly infected bacteria contain 

 not only the two parental phage types but also phage particles 

 with new combinations of characteristics resulting from recom- 

 bination of the parental genetic determinants. This type of re- 

 sult is usual in mixed infection experiments with closely related 

 phages; typical examples have been discussed in Chapter 

 XVIII. The occurrence of genetic recombination implies that 

 the two phage strains are similar enough in genetic constitution 

 that genetic determinants may be interchanged between them. 

 This may be considered to be analogous to the criterion of inter- 

 breeding populations used to define the species category in higher 

 organisms. 



On the basis of the available evidence, one may tentatively 

 propose the use of the results of the mixed infection experiment 

 as a criterion for deciding the relative taxonomic positions of two 

 phage strains with a common host (Adams, 1953b). If the re- 

 sult is mixed growth with genetic recombination, the two strains 

 should be placed in the same species. If the result is mutual ex- 



