WATER 45 



Agar and gelatine media have been most used for the isola- 

 tion, but recently a large number of special media have been 

 advocated for this purpose. While a number of them are use- 

 ful, that of Dieudonne, published in 1909, would appear to be 

 most valuable. It is an alkaline blood agar medium the exact 

 preparation of which is described in the Appendix. 



The plates ought not to be used immediately after their 

 preparation. Dieudonne recommends keeping them for several 

 days in the incubator at 37 C. uncovered and face down, or to 

 heat them for five minutes at 65 C. According to the Report of 

 the International Commission 1 , an equally good result is obtained 

 by keeping them for 48 hours at laboratory temperature. During 

 this time the surface of the agar becomes slightly dry and loses 

 a part of its alkalinity. Once in condition the plates ought to 

 be used within a period not exceeding five or six days. Cholera 

 vibrios grow abundantly on this medium while the bacilli of 

 typhoid fever and dysentery and in particular the B. colt group 

 organisms grow either very badly or not at all. 



On the other hand, B. proteus and B. pyocyaneus, both en- 

 countered frequently in diarrhroeal stools, grow nearly as well as 

 the cholera vibrio and considerably complicate the search. Also 

 many of the non-choleraic vibrios found in water and excreta 

 appear to grow in this medium. According to Dieudonne, how- 

 ever, most of these vibrios are suppressed. 



The colonies of the cholera vibrio on this medium are trans- 

 parent and greyish with a glistening appearance by reflected 

 light but are not definitely characteristic. When many other 

 bacteria are present the identification of the cholera colonies 

 may be very difficult. 



Ordinary nutrient agar is superior to gelatine but the agar 

 must be alkaline. The agar colonies are not very distinctive, 

 being flat discs, transparent and of a grey-blue colour. 



All suspicious colonies on the agar, Dieudonne agar or other 

 medium used, are subcultivated and their characters and proper- 

 ties studied in pure culture. An alternative plan and one which 



1 " Report presented to the permanent committee of the International Office of 

 Public Hygiene in the name of a Commission presided over by Dr Rufier." Trans- 

 lation : U. S. A. Public health, Reports, 1912, vol. xxvn, p. 371. 



