MIXED INFECTION 329 



without by high superinfecting multiphcities of the same phage 

 (Visconti, 1953). This effect seems to develop somewhat more 

 slowly than the exclusion effects described above, but the con- 

 ditions of study were not quite the same in all cases. 



Thus in bacteria infected with T2 and T5 a number of altera- 

 tions in response to superinfecting phage ensue. The various 

 effects are undoubtedly related but immediate causes have not 

 been identified. 



c. Limited Participation 



Another kind of partial exclusion, first described by Dulbecco 

 (1949b), is usually called limited participation. Dulbecco 

 demonstrated it as follows. Bacteria were simultaneously in- 

 fected with 10 to 20 particles of T2 and one or two particles of 

 r mutant per cell. Yields from individual bacteria were scored 

 as mixed or unmixed yields. An appreciable fraction of the 

 yields from the mixedly infected cells always lacked the r marker. 

 Dulbecco interpreted his results in terms of the exclusion of one 

 particle in ten, chosen at random, from participation in growth. 

 Actually, in view of the extreme rapidity of development of 

 partial exclusion of the second kind, it is not clear whether his 

 phenomenon should be distinguished from it. 



A more extreme example of limited participation in phage 

 lambda was reported by E.-L. Wollman and Jacob (1954). 

 Only six particles of this phage seemed to be able to infect a 

 single bacterium. Other experiments with phage lambda 

 appear to be incompatible with this conclusion, however. It 

 must be concluded for the present that limited participation is an 

 ill-defined and doubtful phenomenon. 



3. Summary 



If a bacterial strain is susceptible to two distinguishable phages, 

 it is possible to study the results of mixed infection of single cells 

 with the pair. If the two infecting phages are not related, 

 the usual result is mutual exclusion; one phage or the other multi- 



