APPENDIX 507 



an r+ phage was reinfected with a second r+ phage at least 3 min. 

 after the 1st adsorption, lysis inhibition occurred. The second in- 

 fecting particle might be T2r+, T4r+, or T6r+ regardless of the 

 type of the 1st r+ particle. The phenomenon of lysis inhibition 

 undoubtedly accounts for the small size and turbid halo charac- 

 teristic of the r + plaques. The physiologic mechanism responsible 

 for lysis inhibition is not known. 



The characteristic plaque morphologies are best seen when the 

 phages are plated by the agar layer technique using very soft 



Figure 16. Mixture of phage T2;"^ and T2r plated on strain B, showing 

 inhibition of development of an r plaque by an adjacent r+ plaque. Bright 

 field illumination. 



agar. Luria (personal communication) recommended for this 

 purpose that the base agar be made of 1 .1 per cent agar instead of 

 the 1.5 per cent agar usually used, and that the soft agar for the 

 agar layer be 0.6 per cent instead of 0.7 per cent. With an incu- 

 bation period of 6-8 hr. it is usually possible to distinguish quite 

 sharply between r+ and r plaques, although there may be a few 

 plaques that cannot be definitely classified. 



