10 MICROBIOLOGY 



fulfilled, namely, in places where there are moisture, the 

 required temperature, proper gaseous environment, and a 

 suitable supply of food. These conditions are found in ponds 

 or rivers contaminated with organic matter, such as dead 

 plants or animals, and in soils containing more or less mois- 

 ture together with organic matter. In the habitations of 

 man and beast they are found in the various foods, such as 

 milk and other dairy products, stale meat and cooked vege- 

 tables when not properly refrigerated, and in moist places, 

 in stables and yards for animals. In the animal body they 

 are found on the mucous membrane of the digestive tract, 

 especially of the mouth, and the small and large intestines. 

 There are, however, great differences in the conditions of 

 life and nutrition required by the different species and va- 

 rieties. Bacteria have been reported to multiply in a tem- 

 perature ranging from to 75 C. Some are able to live- 

 in water containing a slight amount of organic substance 

 and others in purely organic matter, such as decomposing 

 vegetables and animal tissues. For some bacteria free oxy- 

 gen is necessary for their multiplication, and the growth 

 of others is promptly stopped by its presence. 



The bacteriological analyses of soils, water, milk, mu- 

 cus from the mouth and intestines of living animals, and de- 

 composing plants and animals, show great differences in the 

 number and varieties of bacteria present. This is accounted 

 for in part by the difference in the food supply furnished 

 by these different substances for the organisms, and in part 

 by the selective power of the bacteria themselves. These 

 facts are very important, as they furnish us with certain 

 data that explain somewhat the existence of bacterial floras. 

 Thus in pure spring water there are very few bacteria, while 

 in water contaminated by soil drainage many organisms are 

 present, often including many species. In uncontaminated, 

 freshly drawn milk, there are certain species of bacteria 

 that come from the milk ducts of the udder where they 

 have become localized and multiply. Again certain species 

 inhabit the deep layers of the skin including the ducts. 



