Structure, Growth and Distribution. 9 



dant supply of moisture. Foods, such as meats, fruits, 

 vegetables and also fodders are protected from the ac- 

 tion of bacteria and molds by drying. Molds require 

 much less moisture for their growth than do the bac- 

 teria. 



Temperature for the growth of bacteria. In common 

 with other forms of plant life, bacterial growth occurs 

 throughout a relatively wide temperature zone ; in fact, 

 wider than in the case of most other plant forms. Most 

 forms of bacteria are checked in their growth when the 

 temperature approximates 40-45 F. although a few of 

 them thrive near the freezing point. At ordinary air 

 temperatures, multiplication of the cells proceeds apace 

 as is shown by the decomposition of organic substances. 

 During hot summer weather, 80-100 F., decomposi- 

 tion is still further hastened. If, however, the tem- 

 perature is increased much above the blood heat, 

 growth of most forms is checked. A temperature of 

 180-140 F. actually kills most of the vegetating bac- 

 teria. To exceed this fatal point, known as the ther- 

 mal death point, is the basis of all methods of preserva- 

 tion of foods by heat. 



Bate of growth of bacteria. When a suitable food is 

 available and temperature conditions are favorable, the 

 bacteria increase in numbers very rapidly. A single 

 organism will divide, and the two daughter cells grow 

 to maturity, ready to divide again, in twenty minutes or 

 less. This rate of growth rarely takes place under na- 

 tural conditions and is never maintained for any con- 

 siderable length of time for most forms of life encoun- 

 ter conditions in nature that restrict their development. 



