THE DIAGNOSIS OF ASIATIC CHOLERA. 479 



In certain doubtful cases the organisms are present in 

 the intestinal canal in very small numbers, and micro- 

 scopic examination is not, therefore, of so much assist- 

 ance. In these cases plates of agar-agar, of gelatin, 

 and cultures in the peptone solution should always be 

 made. 



The plates of agar-agar should not be prepared in 

 the usual way, but the agar-agar should be poured into 

 Petri dishes and allowed to solidify, after which one of 

 the slimy particles may be smeared over its surface. 

 The comma bacillus, being markedly aerobic, develops 

 very much more readily when its colonies are located 

 upon the surface then when in the depths of the med- 

 ium. A point to which Koch calls attention, in con- 

 nection with this step in manipulation, is the necessity 

 of having the surface of the agar-agar free from 

 the water squeezed from it when it solidifies, as the 

 presence of the water interferes with the development 

 of the colonies at isolated points and causes them to 

 become confluent. To obviate this he recommends that 

 the agar-agar be poured into the plates and the water 

 allowed to separate from the surface at the temperature 

 of the incubator before they are used. It is wise, there- 

 fore, when one is liable to be called on for such work 

 as this, to keep a number of sterilized plates of agar- 

 agar in the incubator ready for use, just as sterilized 

 tubes of the media are always ready at hand. The 

 advantage of using the agar plates is the higher tem- 

 perature at which they can be kept, and consequently 

 a more favorable condition for the development of the 

 colonies. 



As soon as isolated colonies appear they should 

 be examined microscopically for the presence of bac- 



