280 MICRO-ORGANISMS IN WATER 



To every 100 c.c. of the water under examination the 

 following addition of nutritive material is made, con- 

 sisting of: 



Gelatine ..... 2 grins. 



Dry peptone . . . . 1 



Sodium chloride . . . . 1 ,, 



Potassium nitrate . . . O10 



Large flasks are employed, so that as extensive a surface 

 as possible of the treated water is exposed to the air. 

 After being preserved for twelve hours at o7 C., a thin 

 pellicle forms on the surface, in which, under the micro- 

 scope, spirillar forms are easily recognisable. According 

 to Sanarelli, spirilla grow so rapidly when thus treated, 

 that if only a few are originally contained in the water, 

 this short time is sufficient to reveal their presence. 

 For their subsequent isolation it is only necessary to 

 take a small piece of the pellicle, and, after mixing it 

 with a little sterile water, to pour gelatine-plates from 

 the dilution. This author states that he has found the 

 presence of a large quantity of albuminoids very un- 

 favourable to the development of spirillar forms ; for 

 this reason, in the preparation of nutritive agar-agar for 

 their subsequent cultivation, he uses, instead of meat 

 extract, ordinary water. In this manner an exception- 

 ally transparent culture-material is procured, which is 

 also especially fitted for the growth of these forms at 

 37 C. 



Thus the crucial tests now recommended by Koch 

 for the differentiation of the cholera bacilli from allied 

 forms with which they are liable to be confounded 

 are : 



1. The positive indol-reaction. 



2. The positive pathogenic effects on guinea-pigs, 

 which are yielded by the cholera bacilli, but apparently 

 not by the allied forms as far as these have been yet 

 examined. 



