CHAPTER XVII 



MIXED INFECTION 



Mixed infection may be defined as the infection of a single 

 bacterium by at least two distinguishable phage particles. 

 This evidently means something quite different from the mixed 

 infection of a higher plant or animal by two distinct viruses. 

 In the latter case the same cells need not be infected by the two 

 viruses and any interaction between them may be very indirect. 

 For this reason what was long known as "interference" between 

 pairs of plant and animal viruses should not be confused with 

 "mutual exclusion" as described below. 



The ratio of infecting phage particles to infected bacteria 

 (the multiplicity of infection) may be varied over a wide range, 

 and independently for the two types of infecting phages. Because 

 one deals with cell populations one cannot determine for any 

 single bacterium exactly how many phage particles of each kind 

 initiated the infection. However, one can calculate by means 

 of the Poisson distribution the proportion of the bacterial popu- 

 lation that was infected with any given combination of phage 

 particles. The time between infections with the two viruses 

 can be varied from virtually zero to an interval as long as the 

 minimum latent period, permitting great flexibility in the design 

 of experiments. 



Mixed infection with two distinct phages may have one of 

 several results. A given cell may liberate both kinds of phage, 

 only one of the two kinds, or neither kind. In addition it may 

 liberate particles differing from either infecting type. The ease 

 with which the significant variables in mixed infections with 

 phages may be controlled has permitted the rapid development 

 of phage genetics. The rapid progress in this field is due to two 

 major factors, the favorable properties of bacteriophages as 



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