26 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



alkaline solution, it readily gelatinises on standing. When 

 the solution of silicic acid is found to give this result, it is 

 cooled, and one-third to one-half of its volume of the mixed 

 alkaline solutions (a and 6) are added, the solutions well mixed 

 and at once poured into Petri dishes or flat-bottomed flasks. 

 The medium should gelatinise in from five to fifteen minutes. 

 The material containing the organisms for examination is 

 introduced and thoroughly mixed, before gelatinisation takes 

 place ; or a streak culture may be made on the surface after the 

 medium has solidified. 



As this method has been used for the study of the very 

 important organisms of nitrification, its method of preparation 

 is of special interest. 



It will be understood that for the study of special organisms 

 various additions to the typical gelatine or agar media can 

 be made. Thus it is characteristic of certain bacteria, especially, 

 e.g., of B. coli, the typical organism of sewage pollution, to 

 produce acid from glucose and other sugars ; when therefore 

 glucose and litmus are added to the medium the reddening of 

 the litmus indicates acid formation. 



The following medium has been suggested by Dr. Mac- 

 Conkey, and has been largely used for the detection of Bacillus 

 coli in polluted water : 



Sodium taurocholate . . . . 0'5 gramme 

 Glucose . . . . . . 05 



Peptone 2'0 



Water .. lOO'O c.c. 



The constituents are heated together, filtered and tinted 

 with litmus solution. The medium is then poured into test 

 tubes and a small inverted fermentation tube placed in 

 each, to serve as a trap for any gas evolved. The tubes are 

 then sterilised in the usual way. 



A certain number of bacteria are found only to develop 

 in absence of air ; such organisms are classed as anaerobic in 



