12 Dairy Bacteriology. 



would be considered as adventive or introduced. Thus, 

 the tubercle bacillus is adventive in milk, even though it 

 maybe derived from the udder direct, while the sour milk 

 bacillus is a natural and therefore indigenous organism. 

 20. Distribution of bacteria. If one classifies bac- 

 teria according to the habitat in which they are found, 

 they may be divided into various groups such as soil, air 

 or water forms. 



1 . Soil bacteria . The superficial layers of the soil teem 

 with myriads of forms of bacteria. The amount of food 

 present and other favorable growth conditions enable 

 many forms to thrive luxuriantly. At a depth of a few 

 feet most of them are filtered out, and the conditions are 

 likewise unsuitable for their growth . so in deep layers the 

 soil is practically sterile. 



2. Air bacteria. The bacteria found in the air are orig- 

 inally from the soil beneath. In the atmosphere they are 

 unable to develop, but exist in a latent condition. Their 

 prevalence in the air is measured by the condition of the 

 soil below and the movement of dust particles. For this 

 reason they are more numerous in summer than in win- 

 ter; the atmosphere of cities contains a larger number of 

 them than country air. They are very prevalent in illy- 

 ventilated houses and out-buildings, particularly barns 

 and stables, where dust from hay and dried manure par- 

 ticles fill the air. 



3. Water bacteria. Water when exposed to the air 

 invariably contains a sufficient amount of organic matter 

 to serve as bacterial food. Some of its germ life is de- 

 rived from dust, or the washings of the land, but many 

 species exist in this element which are not to be found in 

 soil. Stagnant pools rich in organic matter always teem 

 with bacteria, and even running water often has large 

 numbers. The ground water layer is normally free, as 



