694 BACTERIA IN AIR, SOIL, WATER, AND MILK 



pecial value is attached to the organisms which grow at 40° C. since the 

 temperature excludes many non-pathogenic water bacteria, while it 

 permits organisms of the colon-typhoid group to grow. 



The gelatin and agar which are used should be made according to the 

 standard methods recommended by the American Public Health Associa- 

 tion. The gelatin should be made of meat infusion and not of meat ex- 

 tract, and should contain one per cent of Witte's pepton and ten per cent 

 of the best so-called French brand of gelatin. It should not soften when 

 kept at a temperature of 25° C. The agar medium should also be made 

 of meat infusion and of the highest grade of commercial thread agar. 

 For general purposes the standard reaction of media should be one 

 per cent acidity, but for long-continued work on water from the same 

 source the optimum reaction should be ascertained and adhered to, 

 and differences from the standard reaction should be mentioned in the 

 report. 



Incubation of the gelatin should be continued for forty-eight hours 

 in an atmosphere saturated with moisture. When agar is used, incuba- 

 tion for twenty-four hours is usually sufficient, and it is advantageous to 

 employ Petri dishes supplied with porous earthenware covers.^ Simple 

 inversion of the Petri dishes when placed in the incubator obviates the 

 necessity of using the porous covers. 



In counting, the ordinary counting plates divided into 1 cin. squares 

 may be used, but, whenever possible, all the colonies in a plate should be 

 counted. 



The value of the quantitative estimation of bacteria in water is 

 only a comparative one, and no arbitrary standards can be established 

 for the purity of water on this basis. In general it may be said that 

 water containing one hundred bacteria to the cubic centimeter or less 

 is apt to be from a deep source and comparatively pure; that water 

 containing five hundred bacteria to the cubic centimeter or over is 

 open to suspicion, and that any water containing over one thousand 

 to the cubic centimeter is probably from a polluted source. At the 

 same time it is quite impossible to draw any direct conclusions from 

 numerical colony counts, and all such results must be carefully weighed 

 in the balance with qualitative analyses and chemical tests, and knowl- 

 edge of environmental conditions. 



Qualitative Bacterial Analyses of Water. — Of far greater importance 

 than quantitative analysis is the isolation of bacteria either distinctly 

 pathogenic, such as the cholera spirillum and the typhoid bacillus, 



' Hill, Jour. Med. Res., xiii, 1904. 



