590 A MANUAL OF BACTERIOLOGY 



have had such water from a source absolutely free from the possi- 

 bilities of contamination, but usually subsequent examination 

 has revealed the presence of the spores of the B. Welchii. The 

 proximity of manured soil is strongly indicated. 



10. Waters containing the true B. coli and spores of the B. Welchii. 

 These we regard as being decidedly contaminated with faecal matter 

 of recent origin. 



Streptococci. Streptococci are abundant in faeces and 

 sewage, but are extremely rare, if ever present, in unpolluted 

 natural waters ; hence the value of their detection. Strep- 

 tococci as a class are delicate organisms, and it was supposed 

 that their presence indicates recent pollution. 1 Horrocks, 

 on the other hand, believes that they maintain 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 examina- 

 tion of water, but it cannot entirely supplant chemical analysis, 

 which on 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 uncontaminated, the 

 bacteriological examination will occupy three days ; but if con- 

 tamination be present, though it may be presumed 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 deter- 

 mining the characters of the organisms present. 



Media Employed for the Isolation of B. Coli 



(1) Carbolised gelatin. Ordinary nutrient gelatin with the addi- 

 tion 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 



1 Houston, Rep. Med. Off. Loc. Gov. Board for 1898-99. 



