SIGNIFICANCE OF STREPTOCOCCI 715 



their vitality longer even than B. coli, and this is rather 

 the opinion at present. We need further data before we 

 can exactly estimate the value of streptococci as indicators 

 of pollution. There can be no question, however, that, 

 the detection of many streptococci, together with B. coli, 

 indicates serious pollution. 



There can be no doubt of the value of the bacteriological 

 examination of water, but it cannot entirely supplant chemical 

 analysis, which 011 account of its rapidity and the valuable data 

 it yields will probably always remain an integral part of the 

 examination of potable waters. If the water be pure and uncon- 

 taminated, the bacteriological examination will occupy three 

 days ; but if contamination be present, though it may be pre- 

 sumed in the same time, ten days or a fortnight may be required 

 to convert this presumption into a certainty, owing to the length 

 of time necessary for determining the characters of the organisms 

 present. 



Media Employed for the Detection and Isolation 

 of B. Coli 



(1) Carbolised gelatin. Ordinary nutrient gelatin with the 

 addition of 0-05 per cent, of phenol. (Hardly used now.) 



(2) Bile-salt peptone water (MacConkey and Hill). The com- 

 position of this medium is as follows : Sodium taurocholate 0-5 

 grm., glucose or lactose 1-0 grm., peptone 2-0 grm., water 100 c.c. 

 The constituents are dissolved by heating ; the mixture is filtered, 

 and after filtration sufficient neutral litmus solution is added to 

 give a distinct colour. The medium is then distributed into 

 Purham's fermentation-tubes . and sterilised by steaming for 

 twenty minutes on three successive days. The medium may be 

 put up in various sized tubes, a measured volume in each e.g. 

 10 c.c., 20 c.c., 25 c.c., etc., according to the quantity of water 

 which is to be added. For the larger quantities the medium may 

 be made double the above strength. The inoculated tubes are 

 incubated at 37-40 C. for forty-eight hours. The B. coli reddens 

 and ferments both the glucose and lactose media, so that ga.% 

 collects in the fermentation tube. 



(3) Neutral-red broth (Hunter, Makgill, Savage). Th/^ ^ ye 

 known as neutral-red (Griibler's) is reduced by the action of the 



