INTESTINAL BACTERIA 347 



the bacteria which occur in soil also in water. Many of these 

 find this an unsuitable medium for growth and multiplication and 

 soon perish. But some species, among which are B. mycoides, B. 

 subtilis, B. megaterium and B. Mesentericus vulgatus persist for a 

 considerable time. 



Intestinal Bacteria. These are usually of sewage origin. To 

 this class belongs a heterogeneous group of microorganisms which 

 find their way into water from sewage. Many of them are true 

 saprophytes and of themselves are not injurious, but their presence 

 in a water constitutes a danger signal to the bacteriologist. This 

 is especially true of the B. coli group of organisms, the natural 

 habitat of which is the intestinal tract of the higher animal man.- 

 Hence, whenever there is opportunity for these organisms to find 

 their way into waters there may also be opportunity for the patho- 

 gens which cause typhoid fever, cholera and dysenteria to reach 

 the water. It is, therefore, certain that even a little sewage may 

 cause much damage if it enters a water supply for only a few hours 

 at rare intervals, but it is the slight continuous infections which 

 can give rise to a prolonged outbreak of disease. It is well estab- 

 lished that typhoid bacteria die quite rapidly in ordinary waters, 

 and so far as known never multiply in such waters, as is seen from 

 the following (Mills): "To prove whether typhoid-fever germs 

 would survive in the Merrimac River water, when at the low 

 temperature of the month of November, long enough to pass from 

 the Lowell sewers to the service-pipes in Lawrence, a series of 

 experiments was made by the Board by inoculating water from the 

 service-pipes with typhoid-fever germs, and keeping the water 

 in a bottle surrounded by ice, at as near freezing as practicable, 

 for a month and each day taking out one cubic centimeter and 

 determining the number of typhoid germs. The number continu- 

 ally decreased, but some survived twenty-four days. 

 "On the first day there were 6120 germs. 



On the fifth day there were 3100 germs. 



On the tenth day there were 490 germs. 



On the fifteenth day there were 100 germs. 



On the twentieth day there were 17 germs. 



On the twenty-fifth day there were germs." 

 At a higher temperature the life of the organism would have 

 been of even shorter duration. 



Our information in regard to the cholera vibrio is not quite as 

 definite, but experiments indicate that it may multiply to some 

 extent in sterilized river or well water, and that it maintains its 

 vitality in such water for several weeks or even months. 



Natural Purification of Water. Nature's methods of purifying 

 water are mainly: 



