APPENDIX 501 



2. Mixed infection of strain B with even-numbered phages T2, 

 T4, and T6. . . . When pairs of viruses in the immunologically 

 related group T2, T4, and T6 are investigated, it is found that 

 many mixedly infected bacteria liberate both kinds of phage. 

 When mixedly infected bacteria are plated on mixed indicator 

 strains, from 1 to 90 per cent of the plaques are clear. Also, the 

 sum of the plaques on the separate indicator plates is more than 

 the total number of infected bacteria, indicating that a consider- 

 able proportion of infected bacteria liberate both kinds of 

 phage. . . . 



Mutations of Bacteriophages 



The mutant forms of bacterial viruses are likely to play an 

 important role in the development of our ideas about hereditary 

 mechanisms. At present, however, only 3 well-defined types 

 of mutations are known among the bacteriophages: (7) host 

 range mutations; (2) adsorption cofactor mutations; {3) 

 plaque morphology mutations. Considerable effort is being 

 expended in an attempt to extend this list of mutant types and 

 to clarify the biochemical basis for the difference between mutant 

 and wild type phage. 



7. Host Range Mutations 



A host range mutant of a bacterial virus is a variant with an 

 extended host range. Such variants are usually found as a re- 

 sult of plating a large number of bacteriophage particles with a 

 phage-resistant bacterial mutant and picking an isolated plaque 

 if plaques are produced. As an example we can take phage Tl 

 and the phage-resistant bacterial mutant B/l,tryptophanless 

 (see p. 493). If several hundred Tl phage particles are plated 

 with B/l,tr by the agar layer technique no plaques are produced. 

 However, if 10 '^ or 10^ Tl phage particles are plated with B/l,tr 

 a small number of plaques will usually be found. One of these 

 plaques is stabbed with a sterile platinum wire which is then 

 rinsed in a few milliliters of sterile broth. This broth will now 



