INTESTINAL OR COLON-TYPHOID GROUP 287 



plicable; water containing less than 100 bacteria per cubic centi- 

 meter is probably pure; one containing 500 bacteria is suspicious, 

 and one with 1000 bacteria is quite certainly bad. The number of 

 bacteria normally present in unpolluted supplies of various kinds 

 differs considerably; for example, that in the deep wells of a region 

 from those of its lakes, and standards must, therefore, be established 

 for each. Determination of numbers is probably most useful in 

 systematic examination of the efficiency of filtration of public 

 water-supplies. In some countries these tests are made daily, 

 and the maximum bacterial content of the filtered water that may 

 be used has even been fixed by law. 



When gelatin is used, a separate count should be made of the 

 colonies which develop that are capable of liquefying the medium. 

 Such organisms are particularly characteristic of the surface soil, 

 and usually belong to the Bacillus subtilis group. The presence of 

 such in large numbers is an index to the extent of the surface-wash, 

 and not in general of the extent of sewage pollution. This deter- 

 mination is frequently of value in the examination of shallow wells. 



Agar plates may be incubated at blood-heat. The typical 

 water bacteria develop very slowly, if at all, at this temperature. 

 A count made in forty-eight hours of such a plate is a fair index 

 of the amount of sewage contamination usually, as the organisms 

 from this source thrive best at this temperature. This determina- 

 tion, however, is largely displaced by the use of the litmus-lactose- 

 agar plates, as discussed under Qualitative Analysis. 



Qualitative Examination of Water. As has before been stated, 

 water may be examined for the specific pathogens it may contain, 

 or for the presence of sewage and intestinal bacteria, particularly 

 B. coli. 



Isolation of Specific Pathogens. B. typhosus is the organism 

 for which examinations have been most frequently made. It has 

 been actually isolated from water in a few instances only. Fre- 

 quently only a very small percentage of the colonies which develop 

 from direct plating of typhoid stools are typhoid colonies. The 

 chances of direct isolation by plating sewage or water from a sup- 

 ply is, therefore, remote, even though this organism be present in 

 numbers such as to cause an epidemic of the disease among the 

 consumers. 



