BACILLUS TYPHOSUS. 435 



8. To the filtrate add 1 per cent, of potassium 

 iodide. (Use a solution so made that 1 c.c. is equiva- 

 lent to 1 gramme of the salt.) 



9. Decant into tubes and sterilize. 



One of the most important points in working with 

 this medium is that the incubator must be kept at a con- 

 staut temperature of from 22 to 24 C. If the plates 

 be put away before the gelatin has thoroughly cooled, 

 or if the room becomes a very little too warm at any 

 time during the colony growth, so as to soften the gel- 

 atin, both the typhoid and the colon bacilli will develop 

 threads or become oval, and thus the characteristic dif- 

 ferentiation will be lost. Care must be taken, also, 

 that the room in which the plates are examined be not 

 too warm. This causes great inconvenience during the 

 summer months in most parts of the United States, 

 and requires special methods for keeping a room at the 

 proper temperature and keeping the plates cool during 

 their examination. 



The iodide of potash prevents the great majority of 

 bacteria, especially the liquefiers, from developing; in 

 fact, little but colon and typhoid bacilli appear on the 

 plates. This is one of the chief advantages of the 

 medium in the examination of both water and feces. 



Appearance of the Colonies. The colon is the first to 

 develop; the colonies are rough and granular in appear- 

 ance and greenish-brown in color; for the greater part 

 the colonies are on or near the surface. The typhoid 

 develop later, and their colonies usually show the clas- 

 sical " dew-drop " appearance small, white, gleaming, 

 generally without variation in substance, but occasion- 

 ally slightly granular. This point causes some trouble 

 to one first using the medium, as the young colon colo- 



