502 BACTERIOPHAGES 



contain a mixture of the parent Tl phage particles which were 

 originally put on the plate and the new mutant form of Tl. 

 Several dilutions of this broth suspension are now plated on B/1,- 

 tr and plaques of the mutant will be produced. Two replatings 

 of this mutant phage in this manner suffice to free it completely 

 from the parent Tl phage since the latter does not multiply on 

 B/l,tr. When the mutant of Tl has been purified a stock may 

 be prepared by growing it on B/l,tr. This stock produces 

 plaques identical in morphology with the parent Tl, and there is 

 no detectable difference in immunologic properties between the 

 parent and the mutant. However, the mutant forms plaques 

 on B/l,tr whereas the parent does not. The mutant is called 

 T\h and the parent strain is then called Tl/z+ to indicate that it 

 is the wild type with respect to this particular character. 



Table XXV shows the host ranges of T1A+ and Tl^ and, for 

 comparison, of T5. The host ranges of Tl A and T5 are identical, 

 although these phages may be readily distinguished by plaque 

 morphology and by immunologic properties. It is obvious that 

 the host range of a virus is of no value in the primary classification 

 of the virus. However, if the immunologic and morphologic 

 properties are known, the host range is of further value in sub- 

 classification of the virus. 



TABLE XXV 



Host Ranges of T1A+, Tlh, and T5 



When plated on T\h+ TU T5 



B Plaques Plaques Plaques 



B/l,tr No plaques Plaques Plaques 



B/1,5 No plaques No plaques No plaques 



If TIA is plated on B/l,tr and the corresponding phage-resist- 

 ant bacterial mutant isolated as previously described (p. 490), 

 this mutant will be found to be B/l,tr/l,5 ; i.e., the mutant is re- 

 sistant to Tl/j+, Tlh, and T5 and deficient in ability to synthe- 

 size tryptophan. No host range mutant of Tl capable of at- 

 tacking B/1,5 has been found. Similar host range mutants have 



