280 MICRO-ORGANISMS IN WATER 



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

 following addition of nutritive material is made, con- 



Large flasks are employed, so that as extensive a surface 

 ^s 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. 

 Por 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° a 



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. 



