ORGANISMS OF CONTAMINATION 57 



term the typical bacillus and closely allied organisms. When 

 this bacillus can be detected in a small measured quantity of 

 water, that is to say, in 1, 2 or 3 c.c, it is assumed (a) that the 

 organism has gained access to the water from sewage, and (J) that 

 recently, (c) It is further assumed that certain disease-producing 

 bacteria which occur frequently in sewage may also be present in 

 the water, though if present at all in the water, in considerably 

 smaller numbers than B. coli. (d) Further, judging the matter 

 broadly, the higher the number of R coli the heavier will have been 

 the recent sewage pollution, and the greater the probabihty of the 

 presence of disease-producing bacteria. Conversely, if B. coli is 

 not present, one may assume with some probability of being correct, 

 that such disease-producing bacteria as the bacillus of typhoid fever 

 will also be absent, and that the particular sample of water under 

 examination might safely be used for drinking purposes. 



There is difference of opinion as to the exact quantity of a water 

 which must be free from a single specimen of B coli in order that it 

 may be said that the sample is a " safe " one ; but many would in 

 practice accept the standard 1 or 2 c.c. 



It has already been stated that the presence of B. coli in a water is 

 not of importance, because this organism itself, under the ordinary 

 conditions, is Hkely to be harmful, but rather because it serves as an 

 index of sewage or surface pollution. In this connection it may be 

 said that a single examination of a water is of practically no value 

 when the results of the bacteriological examination are favourable ; 

 it is only after repeated examination has shown that B. coli is absent 

 from the water for a prolonged period, and after local inspection has 

 shown that there are no possible sources of dangerous sewage con- 

 tamination, that one is justified in giving a positive opinion as to 

 the safety of a water. On the other hand, a single bacteriological 

 examination with an unfavourable result will prove the actual 

 occurrence, and suggest the possible recurrence, of sewage contamina- 

 tion, and will necessitate renewed inspection if no obvious source of 

 contamination is known to exist. 



B. coli is commonly considered as evidence of contamination by 

 sewage, but it is possible for the bacillus to gain access to the water 

 from other sources also. The bacillus is present in the excreta of 

 mammals generally, and has been found in the excreta of birds, and 

 in surface waters there will undoubtedly be a certain amount of 

 contamination caused in this way. The question as to whether any 

 contamination of this kind can be caused by various fishes, and other 

 forms of aquatic life, is not fully established, though Eyre has 

 recently found the B. coli in the excreta of iishes, as well as mammals 

 and birds.* 



* Lancet, 1904, i., p. 648. 



