30 VETERINARY HYGIENE 



contain bacteria. Waters from deep wells and certain springs con- 

 tain very few bacteria, 100 or less per c.c., some less than 10 per c.c., 

 but the majority of wholesome drinking waters contain many more 

 than this. Enumeration of the number of bacteria in filtered waters 

 forms a valuable and safe guide as to the efficiency of the filtration, 

 and it is considered that if the number does not exceed 100 per c.c. 

 the filtration is in every way satisfactory. The examination 

 must be made immediately the water is drawn, as after the lapse 

 of a few hours the bacteria multiply enormously. Water drawn 

 from the main or from house taps does not necessarily contain as 

 low a count as at the source of supply, that is, as it leaves the 

 filter-beds. 



As to the total number of bacteria that should be permitted in 

 drinking water, irrespective of their nature, it is very questionable 

 if any standard of value can be adopted, though 100 per c.c. has been 

 suggested as the limit. 



There is great variation in the bacterial content of unfiltered 

 waters. An average well may contain anything from 100 to 2000 

 per c.c., while in what may be regarded as first-class waters from 

 springs, deep wells and well-protected shallow wells sunk in suitable 

 ground, the number should not be expected to exceed 1000 per c.c. 

 Moorland streams are usually comparatively free from bacteria, but 

 most of them contain more than good springs and well-constructed 

 deep wells do. Wells in constant and regular use contain 

 fewer bacteria than those in which the water lies stagnant for 

 periods. Sewage may contain from six to twelve million bacteria 

 per c.c. 



Of the micro-organisms for which special search is made, there 

 are bacilli of the colon group, streptococci, and Bacillus enteriditis 

 sporogenes. When pathogenic organisms get into water in com- 

 pany with the B. coli and its allies they die out (owing to unsuitable 

 environment) before the latter, so that the absence of B. coli is 

 more or less definite evidence that pathogenic organisms are also 

 absent. On the other hand B. coli has been found in considerable 

 numbers in water of undoubted purity of origin and safe from 

 accidental contamination. B. coli is found in crude sewage in 

 numbers ranging from 100,000 per c.c. upwards. 



The presence of streptococci in water is generally regarded as 

 an indication of recent contamination with sewage as they soon die 

 out in water; on this, however, opinions are divided. Dr. Thresh 

 considers that their presence, per se, is of no significance, but if with 

 colon bacilli there is reason for strongly presuming that there has 

 been pollution with sewage or manurial matter, and that if the 



