100 BACTERIOLOGICAL METHODS 



the streptococci), it is desirable to examine such colonies micro- 

 scopically and to inoculate into other media and perhaps to 

 test for indol formation, in order to obtain satisfactory proof as 

 to whether or not they are colon bacilli. 



Neutral red (a safranine dye) reduction was at one time con- 

 sidered a very important check test for the colon group. Stokes, 

 as early as 1904, recommended that neutral red be added to lactose 

 broth in the fermentation tubes which contain the required dilu- 



FIG. 27. B. coli showing flagellae stained by the van Ermengen method (X 

 1000). (MacNeal, from McFarland after Migula.) 



tions of the liquids to be examined. 30 to 50 per cent, gas forma- 

 tion in the closed arms of the tubes and the change of the neutral 

 red to canary yellow, is said to be characteristic for the colon 

 group. It would appear that the majority of bacteriologists are 

 inclined to omit the neutral red test as being of little value. 



The production of indol in peptone broth or solutions is another 

 colon bacillus test much used in the United States. Boehmes' 

 modification of the Ehrlich method is now generally employed, 

 made as follows: Two solutions are required. 



