498 BACTERIOPHAGES 



Tl plaques may not be distinguishable from the background ex- 

 cept for the overlaps, so the plates should be examined after 5 or 6 

 hr. of incubation. A plate of mixed viruses on mixed indicator 



strains is illustrated in Fig. 14. 



Figure 14. Mixture of phages Tl and T7 plated on mixed indicators B/1,5 

 and B/3,4,7. Note the 2 kinds of turbid plaques and clear overlapping areas 

 of complete lysis. Bright field illumination. 



Applicaliun. A most important use of mixed indicator strains 

 is in analysis of the phage yields from mixedly infected bacteria. 

 If a bacterium is mixedly infected with 2 different viruses and 

 then plated on the appropriate mixed indicator strains before 

 the end of the latent period, 2 possible plaque types might be 

 found. If the infected bacterium liberates both kinds of phages, a 

 clear plaque will be formed. This is illustrated in the following 

 examples. 



