Isolation of the Colon Bacillus. 99 



come blue again after forty-eight hours', owing to the for- 

 mation of amines and ammonia by the action of the 

 bacteria upon the nitrogenous matter present. If the 

 dilution is too low, the resulting colonies will be small 

 and imperfectly developed, making it difficult to be sure 

 of pure cultures for the subsequent tests. A great 

 number of colonies will also prevent the change of reac- 

 tion from acid back to alkaline. Since many bacteria 

 ferment lactose with the formation of acid, it is erroneous 

 to regard all colonies as those of B. coli; several col- 

 onies from each plate should be isolated upon agar 

 streaks and farther studied in subculture. 



In the selection of those red colonies which are to be 

 fished from the litmus-lactose-agar plate the appearance 

 of the growths must be closely noted. A colony of irregu- 

 lar contour, surrounded by a very faint area of reddening, 

 will probably belong to some member of the B. mycoides 

 group (Winslow and Nibecker, 1903); small, compact, 

 bright-red colonies are characteristic of the streptococci, 

 and Gage and Phelps (Gage and Phelps, 1903) have 

 pointed out that of these there are two types, one of a 

 brick-red color, and of such consistency as to be readily 

 picked up by the needle-point, and the other smaller and 

 of an intense vermilion color. The colonies of the colon 

 bacillus are usually well formed, pulvinate on the sur- 

 face and fusiform when growing deeper down. 



If no red colonies appear on the litmus-lactose-agar 

 plate one of three things has occurred. There may be 



