138 BACTERIA IN WATER 



bacteria present in rivers vary so greatly that there is little use 

 in quoting figures, most information being obtainable by 

 comparative enumerations before and after a given event has 

 occurred to a particular water. Such a method is thus of great 

 use in estimating the efficacy of the filter beds of a town water 

 supply. These usually remove from 95 to 98 per cent of the 

 bacteria present, and a town supply as it issues from the filter 

 beds should not contain more than 100 bacteria per c.c. Again, 

 is it found that the storage of water diminishes the number of 

 bacteria present. The highest counts of bacteria per c.c. are 

 observed with sewage ; for example, in the London sewage the 

 numbers range from six to twelve millions. 



Much more important than the mere enumeration of the 

 bacteria present in a water is the question whether these include 

 forms pathogenic to man. The chief interest here, so far as 

 Europe is concerned, lies in the fact that typhoid fever is so 

 frequently water-borne, but cholera and certain other intestinal 

 diseases have a similar source. The search in waters for the 

 organisms concerned in these diseases is a matter of the greatest 

 difficulty, for each belongs to a group of organisms morpho- 

 logically similar, very widespread in nature, and many of which 

 have little or no pathogenic action. The biological characters 

 of these organisms will be given in the chapters devoted to 

 the diseases in question, but here it may be said that from 

 the public health standpoint the making of their being 

 found a criterion for the condemning of a water is impractic- 

 able. There is no doubt that the typhoid and cholera 

 bacteria can exist for some time in water at least this 

 has been found to be the case when sterile water has been 

 inoculated with these bacteria. But to what extent the same is 

 true when they are placed in natural conditions, which involve 

 their living in the presence of other organisms, is unknown, for 

 it may be safely said that by no known method can the presence 

 of either be demonstrated in the complex mixtures which occur 

 in nature. With regard to the typhoid bacillus, of late the 

 tendency has been to seek for the presence of indirect bacterio- 

 logical evidence which might point in the direction of the 

 possibility of the presence of this organism. The methods 

 employed and the lines along which such investigations have 

 gone have already been alluded to in connection with soil. 

 The whole question turns on the possibility of recognising 

 bacteriologically the contamination of water with sewage. 

 Klein and Houston here insist on the fact that in crude sewage 

 the b. coli or the members of the coli group are practically 



