QUALITATIVE BACTERIOLOGICAL ANALYSIS. 99 



B. coli are intended either to repress the growth of the ordinary 

 water-bacteria, so as to enable the B. coli to develop without 

 the chance of being crowded out by these microbes, or tc 

 provide a medium which will be so favourable to the growth 

 of B. coli that it will develop in advance of the competing 

 organisms. 



The methods which are commonly used are as follows : 



(1) Parietti's method. In this plan a series of broth tubes 

 are taken and inoculated with gradually increasing quantities of 

 the following mixture : 



Carbolic acid ...... 5 grammes 



Hydrochloric acid (pure) .... 4 grammes 



Distilled water 100 c.c. 



As a result of the admixture the broth tubes contain from 

 0'05 to 0*3 per cent, of carbolic acid. Many bacteriologists 

 omit the hydrochloric acid from the solution, and do not use 

 the tubes which contain more than from 0'15 to 0*2 per cent, 

 of carbolic acid. When the B. typhosus is supposed to be 

 present with B. coli it is wise to employ only the tubes which 

 contain 0'05 per cent, of carbolic acid; the reason for this 

 will be explained under the section devoted to B. typhosus. 

 The broth tubes are then seeded with varying quan- 

 tities of water, and incubated at 37 C. All the tubes which 

 show growth after one to three days incubation at 37 C. are 

 then plated out in ordinary or carbolic acid (0*05 per cent, 

 gelatine, and the colonies which develop are " fished " and 

 submitted to the various tests described under B. coli. In my 

 own experiments I found that B. coli, freshly isolated from 

 stools, would grow in 0*2 per cent, carbol-broth but if the bacillus 

 were allowed to remain in sterile sewage for six weeks it would 

 not grow in more than O'l per cent, carbol-broth. 



(2) Vincent suggested cultivating the seeded broth tubes at 

 4& C., at which temperature the B. coli will grow, but water 

 organisms are either unable to develop or do so very feebly. 



(3) Fakes recommends that the water should be cultivated 

 under anaerobic conditions (Buchner's tube or hydrogen gas) in 

 glucose-formate broth (2 per cent, glucose, 0*4 per cent, sodium 

 formate) at 42 C. 



