BACTERIA IN WATER 161 



season there is most spring water entering, and very little water 

 as washings off land. In the case of other rivers the bacteria 

 have been found to be fewest in winter. A great many circum- 

 stances must therefore be taken into account in dealing with 

 mere enumerations of water bacteria, and such enumerations 

 are only useful when they are taken simultaneously over a 

 stretch of river, with special reference to the sources of the 

 water entering the river. Thus it is usually found that im- 

 mediately below a sewage effluent the bacterial content rises, 

 though in a comparatively short distance the numbers may 

 markedly decrease, and it may be that the river as far as 

 numbers are concerned may appear to return to its previous 

 bacterial content. The numbers of 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, it is found 

 that the storage of water effects a very marked bacterial purifica- 

 tion. Thus Houston has shown in one series of observations 

 that while 93 per cent, of samples of raw river Lea water 

 contained b. coli. in 1 c.c. or less, in the stored water 62 per 

 cent, of the samples showed no b. coli to be present in 100 c.c. 

 According to Coplans, however, the diminution is not necessarily 

 clue to the organisms being killed ; the real cause may be the 

 agglutination of the bacteria following on changes in the electric 

 conductivity which take place in stored water. 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 most important of these is the 

 typhoid bacillus, though the b. dysenterise, the organisms of the 

 paratyphoid group, the b. enteritidis sporogenes, and, in certain 

 circumstances, the cholera vibrio, must also be kept in mind. 

 On account of the small numbers which may be present in a 

 dangerous water, the direct isolation of these organisms is a 

 matter of great difficulty (though it is possible by the methods 

 described in Chapter XV. ), and from the public health standpoint 

 the making of their being found a criterion for the condemning 

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