446 BACTERIOPHAGES 



Soft agar for the agar layer technique contains only 7 g. of Difco 

 agar /liter and is most conveniently put up in 50 ml. amounts in 

 bottles, to be melted and used as needed. 



These media are probably suitable for use with any of the 

 enteric bacteria. For more fastidious host organisms special 

 media may have to be devised. 



For certain types of work with bacterial viruses it is desirable 

 to grow the host organisms on chemically defined media. 

 [The composition of two simple media used in culturing E. 

 coli are given below: 



" Bring to pH 6.8 with NaOH. 

 " Autoclave separately. 



Numerous minor modifications of these media will be found in 

 the phage literature.] The media may be enriched if desired 

 with various amino acids, growth factors and salts to support 

 growth of more fastidious organisms. E.g., strain B of E. coli 

 will grow abundantly in this medium, but two of its phage- 

 resistant mutants will not grow unless certain amino acids are 

 added. Also, most phages of the T group are produced in high 

 titer on strain B of E. coli when grown on F medium, but T5 

 is not produced unless CaCh to a concentration of 0.001 A^ 

 is added. With such a chemically defined medium it is possible 

 to study the eflfect of the nutritional environment of the host 

 cell on the yield of phage. 



With both broth and chemically defined media, maximal 

 growth of E. coli is achieved only with active aeration of the 



