262 MICROORGANISMS IX WATER 



it is precisely by such direct experiments on animals 

 that nearly all the above-mentioned pathogenic bacteria 

 have been discovered in water, the animal system serv- 

 ing to sift out, as it were, the few pathogenic indivi- 

 duals from an overwhelming majority of harmless forms 

 with which the former must invariably be accompanied 

 in natural waters. In the case of the microbes of 

 typhoid and cholera then, since the animal test is im- 

 possible, recourse must be had for their detection and 

 isolation from water to the far more laborious and 

 less delicate method of artificial cultivation. In con- 

 sequence, however, of the general enormous preponder- 

 ance of the common water bacteria, the ordinary process 

 of gelatine -plate cultivation will only, in the most excep- 

 tional cases, lead to the detection of the pathogenic 

 forms, and special methods have had to be devised in 

 which the growth and multiplication of the latter is 

 favoured and stimulated, whilst the proliferation of the 

 other bacteria present is either retarded or inhibited 

 altogether. It is precisely in connection with such 

 special methods for the discovery of particular forms 

 that some of the most important advances have recently 

 been made in the bacteriology of water. 



Koch, 1 whilst engaged upon investigating the cause 

 of cholera in India, discovered cholera bacilli in a tank 

 which was used for drinking purposes. An epidemic 

 of cholera had broken out in a small village in the 

 neighbourhood of Calcutta, and the attention of the 

 cholera commissioners was attracted to the tanks which 

 not only supplied water for drinking purposes, but in 

 which the natives bathed and also washed their clothes, 

 and to which, in addition, sewage gained easy access. 



1 Berl. Uinische Wochensclirift, 1884, Xos. 31, 32, 32a ; also ' Bericht 

 iiber die Thiitigkeit der Cholera-Kommission,' Arbeit en <t. d. kaiscrliclicn 

 Gcsiindlicitsamtc, vol. iii., 1887, p. 182. 



