A SYNTHETIC MEDIUM FOR THE DIRECT 



ENUMERATION OF ORGANISMS OF 



THE COLON-AEROGENES GROUP 



S. HENRY AYERS and PHILIP RUPP 



From the Research Laboratories of the Dairy Division, United States Department 



of Agriculture 



Received for publication June 12, 1918 



The direct numerical determination of the organisms of the 

 colon-aerogenes group has attracted the attention of numerous 

 investigators and several different media have been originated 

 for this purpose. There are many difficulties which are encoun- 

 tered in an attempt to devise a selective medium upon which 

 organisms of this group will grow while other species of bacteria 

 are excluded. The difficulties are too well recognized by all who 

 have attempted direct quantitative determinations to warrant 

 further discussion. 



The media commonly used for the direct determination of the 

 colon-aerogenes group are very complex and depend largely 

 for their success upon some constituent which inhibits the non- 

 gas forming lactose fermenting organisms. This seems to be 

 the wrong principle. Instead of depending upon inhibition of 

 certain species it is believed that it is far better to simplify the 

 medium to a point where the sources of nitrogen and carbon are in 

 such a form as to allow only the growth of the organisms desired. 

 With this view in mind a simple synthetic medium has been 

 devised in which there is a single source of nitrogen, namely, 

 sodium ammonium phosphate and a single source of carbon, 

 namely, lactose. 



COMPOSITION OF THE MEDIUM 



Solution I 



per cent 



Sodium ammonium phosphate 0.4 



Acid potassium phosphate 0.2 



Lactose 1.0 



Dissolve in distilled water. 



433 



NEW YO«fc 

 eOTANfCAL 



THE JOURNAL OF BACTEBIOLOGT, VOL. Ill, NO. 5 



