14 THE BOOK OF CHEESE 



20. Bacteria in the milk. Bacteria are microscopic 

 unicellular plants, without chlorophyll. Besides bacteria, 

 there are other forms of the lower orders of plants found 

 in milk, such as yeasts and molds. While the bacteria 

 are normally the more important, frequently yeasts and 

 molds produce significant changes in milk and other 

 dairy products. Bacteria are very widely distributed 

 throughout nature. They are so small that they may 

 easily float in the air or on particles of dust. Many 

 groups of bacteria are so resistant to adverse conditions 

 of growth that they may be present in a dormant or spore 

 stage, and, therefore, not be easily recognized ; when 

 suitable environments for growth are again produced, 

 development begins at once. They are found in all 

 surface water, in the earth and upon all organic matter. 

 There are a great many different groups of bacteria; 

 some are beneficial, and some are harmful. As they are 

 so small, it is difficult to differentiate between the bene- 

 ficial and harmful kinds, except by the results produced, 

 or by a careful study in an especially equipped laboratory. 

 The bacteria multiply very rapidly. This is brought 

 about by fission ; that is, the cell-walls are drawn in at 

 one place around the cell, and when the walls unite at the 

 center, the cell is divided. There are then two bacteria. 

 In some cases, division takes place in twenty to thirty 

 minutes. Like other plants, they are very sensitive to 

 food supply, to temperature and to moisture, as con- 

 ditions of growth. Inasmuch as the bacteria are plant 

 cells, they must absorb their food from materials in 

 solution. They may live on solid substances, but the 

 food elements must be rendered soluble before they can 

 be used. Most bacteria prefer a neutral or slightly acid 

 medium for growth, rather than an alkaline reaction. 



