488 JAMES M. SHERMAN 



halo which surrounds their colonies. This differentiation is 

 much clearer on plates containing lactose or glucose agar than on 

 those with plain agar, as the opacity of the medium is very much 

 increased in the presence of a fermentable carbohydrate, due to 

 the growth of acid-producing organisms. In fact, unclarified 

 lactose agar gives, in our hands, nearly as clear a differentiation 

 of bacterial types as does the special casein agar devised by 

 Ayers.^ These points are well illustrated by the following 

 photographs. 



SUMMARY 



Data are presented which it is thought warrant the recommen- 

 dation that agar containing glucose or lactose be adopted as the 

 standard medium for the routine bacteriological analysis of milk. 



The advantages of a carbohydrate medium over plain agar 

 may be summed up in the following points : 



1. A higher count is obtained. 



2. The colonies grow larger and more rapidly which greatly 

 facilitates the counting of plates. 



3. It is of some value as a differential medium. 



* U. S. Bureau of Animal Industry, Report for 1911. 



