BACTERIA AND THEIR ROLE IN NATURE 15 



occur only in small numbers or not at all in deep wells and 

 springs. They are numerous in shallow wells, ponds, and streams 

 draining inhabited districts. Often one cubic centimeter of water 

 will contain millions. They are even found in salt lakes and the 

 ocean. In these waters they function even at great depths. The 

 kind and number vary with the composition of the water and 

 the original contamination. Great Salt Lake with its large quan- 

 tities of common salt contains few organisms, whereas the Great 

 Lakes in the central part of the United States contain many. A 

 turbid stream like the Mississippi which contains the drainage 

 from many cities has a great variety and number of bacteria in 

 opposition to the clear, rapid, flowing water of uninhabited 

 mountainous regions. 



Milk, as secreted by the cow, is practically free from bacteria, 

 but when it is drawn from the udder it always contains bacteria. 

 The number and kind vary with specific cows. Some healthy 

 cows may give milk with as few as twenty bacteria in a cubic 

 centimeter, whereas others may give milk with thousands, and 

 diseased cows millions. During the milking process milk receives 

 germs from the coat of the cow, the clothing of the milker, the 

 buckets, strainers, and other things with which the milk comes in 

 contact. Often by the time it reaches the consumer it contains 

 millions of microbes in every cubic centimeter. 



Bacteria are found on the surface, but not on the inside of un- 

 damaged fruit, and vegetables. In short, all food except that re- 

 cently cooked contains bacteria, the number and kind varying with 

 the nature and age of the food. 



Living as we do in a world which is filled with bacteria, we 

 can expect to find them on the surface of the skin and mucous 

 membranes. Usually they live there feeding on the foreign mat- 

 ter of the surface. At times they work down into the pores of 

 the skin and manifest themselves as blackheads, or as the more 

 painful affliction — ^boils. Normally the infant enters the world 

 free from bacteria, but bacteria soon settle on the skin. They 

 penetrate the nose and mouth ; the first respiratory movements and 

 cries carry them into the respiratory passages 5 and between the 



