ORGANISMS OF THE COLON-AEROGENES GROUP 435 

 INCUBATION 



Plates may be incubated at 30°C. or at 37°C. and should be 

 counted after forty-eight hours. The colonies appear somewhat 

 more quickly and are larger at the higher temperatures. If the 

 plates are incubated longer than forty-eight hours some bacteria 

 which do not belong to the colon-aerogenes group may develop. 

 Upon incubation the medium becomes red, but there is no diffi- 

 culty in distinguishing the characteristic deep red colonies. 



APPEARANCE OF COLONIES OF ORGANISMS OF THE COLON-AEROGENES 



GROUP 



The colonies appear on the plates as medium sized red colonies 

 with a deep red ring around them. The color about the colony 

 is quite characteristic, although some have only a deep red color 

 in the colony itself. 



Bacteria which may also grow upon the medium and which 

 do not belong to the colon-aerogenes group usually give pink or 

 uncolored colonies which can be readily distinguished from those 

 of the gas formers. Before this medium is used for routine work 

 it is recommended that pure cultures of the organisms of the 

 colon-aerogenes group be plated on the niedium in order that 

 the appearance of their colonies may be observed. 



SELECTIVE ACTION OF THE MEDIUM 



The synthetic medium has a selective instead of inhibitory 

 action for two reasons. First, it allows only the development of 

 bacteria which can obtain their nitrogen from a very simple 

 compound, such as sodium ammonium phosphate. This natur- 

 ally prevents the growth of a large number of species of bacteria. 

 Second, the only source of carbon is lactose which therefore 

 allows only the growth of bacteria which ferment this sugar. 



A large number of organisms of the colon-aerogenes group have 

 been tried and all found to be capable of utilizing nitrogen and 

 carbon from the above mentioned sources. Relatively few 

 other species of bacteria were encountered which could grow on 



