508 BACTERIOPHAGES 



If a typical stock of T2r~^ phage is diluted and plated so as to 

 have about 10^ plaques/plate and is examined after 8 hr. incu- 

 bation, 3 distinct types of plaques (Fig. 15) should be found: 

 (a) Most of the plaques will be typical /•+ plaques, quite small 

 and with a turbid halo, (b) A few plaques will be typical r 

 plaques, larger with a sharply defined clear halo, (c) About 1 

 per cent of the plaques will be mottled, i.e., will be primarily r + 

 type, with a sector of complete lysis owing to the appearance of 

 an r mutant during development of the plaque. If an r+ plaque 

 overlaps an r plaque, there is a sharp boundary where lysis inhi- 

 bition by the r+ phage has prevented development of the r plaque 

 (Fig. 16). 



The r mutant can be readily isolated by stabbing either an r 

 plaque or r sector in a mottled plaque with a sterile platinum 

 wire and rinsing the wire in broth. If appropriate dilutions of 

 this broth are then plated by the agar layer technique, many 

 typical r plaques should be found. A well isolated r plaque can 

 then be stabbed and used to inoculate a broth culture of B to pro- 

 duce an r stock. 



Mixed Infection and Genetic Recombination 



Mixed Infections with r+ and r Variants of Same Phage Type 



Hershey (1946a) made mixed infections of B with T2r"'' and 

 T2r. The input ratio of phage to bacteria was such that the 

 multiplicity of infection was about 2.5 for each variant. Under 

 these conditions about 85 per cent of the bacteria should have ad- 

 sorbed at least 1 phage particle of each kind ; i.e., 85 per cent 

 should be mixedly infected. The free phage was neutralized 

 with antiserum and platings were made on B from F.G.T. before 

 the end of the latent period and from S.G.T. after all bursts had 

 taken place. In the plate before the end of the latent period the 

 plaque morphologies were carefully examined and showed that 

 14 per cent of the plaques were r+, 21 per cent r, and 65 per cent 

 mottled. The mottled plaques were due to mixedly infected bac- 

 teria which liberated both T2r+ and T2r phages. In this ex- 



