BACTERIA IN WATER. 645 



and numerous observations show that the number of bacteria in river 

 water is greatly increased in the vicinity of and below the mouths 

 of city sewers. 



We conclude from the experimental data recorded that water 

 containing less than 100 bacteria to the cubic centimetre is presum- 

 ably from a deep source and uncontaminated by surface drainage, 

 and that it will usually be safe to recommend such water for drink- 

 ing purposes, unless it contains injurious mineral substances. 

 Water that contains more than 500 bacteria to the cubic centimetre, 

 although it may in many cases be harmless, is to be looked upon 

 with some suspicion, and water containing 1,000 or more bacteria is 

 presumably contaminated by sewage or surface drainage and should 

 be rejected or filtered before it is used for drinking purposes. But, 

 as heretofore stated, the danger does not depend directly upon the 

 number of bacteria present, but upon contamination with pathogenic 

 species which are liable to be present in surface water and sewage. 

 In swallowing a glassful of pure spring water a number of bacteria 

 from the buccal cavity are washed away and carried into the stomach, 

 which, if enumerated, would doubtless far exceed in numbers those 

 found in the most impure river water. 



The number of bacteria does not depend alone upon the amount 

 of organic pabulum contained in a water, and cannot be depended 

 upon in forming an estimate of this ; for, as has been shown by 

 Bolton, certain water bacteria multiply abundantly in water con- 

 taining comparatively little organic matter, while other species fail 

 to grow unless the quantity is greater. In a water containing con- 

 siderable nutrient material the water bacteria may be restrained in 

 their development by other species present until the amount of pabu- 

 lum is reduced so that these no longer thrive, when the common 

 water bacteria will take the precedence, and an enumeration may 

 show a greater number of colonies than at first. But, in general, 

 water rich in organic material contains a greater number of bacteria 

 and a greater variety of species than that which is comparatively 

 pure. 



That certain bacteria may multiply in water which has been 

 carefully distilled has been shown by Bolton and others. Two com- 

 mon water bacteria Micrococcus aquatilis and Bacillus erythrospo- 

 rus multiplied abundantly in doubly distilled water, and when 

 this water was again sterilized and re-inoculated with one of these 

 species the same abundant increase occurred. This was repeated six 

 times with the same result (Bolton). Computing the number of 

 these water bacteria in ten cubic centimetres of distilled water at 

 twenty millions, and estimating their specific gravity at one, and the 

 diameter of the individual cells at one /', the total weight of the entire 



