10 Agricultural Bacteriology. 



Food is not always abundant, the temperature may be- 

 such as to limit or even stop growth. Every living thing 

 has its enemies. These limitations check bacterial growth 

 in the same manner as all the higher forms of life are 

 checked. 



Under artificial conditions the bacteria may grow 

 very rapidly for a time. For instance, it has been 

 shown that a single organism when placed under the 

 most favorable conditions as to food and temperature 

 will increase in ten hours to the enormous number of 

 1,240,000,000. This rapid growth does not continue, 

 but, on the other hand, goes on more and more slowly 

 and at last ceases altogether, because the products 

 formed by the organisms] themselves accumulate and 

 make further growth impossible. Every form of life 

 living on plant or animal matter may be killed by its 

 own excretions unless they are removed. The bacteria 

 are no exception to this rule. 



Effect of cold on bacteria. Low temperatures retard 

 the rate of growth of the bacteria. The exact point at 

 which growth ceases varies widely with different spe- 

 cies. Some forms as the organism causing tuberculosis 

 will not grow below 90 F. Still others will grow at 

 temperatures below freezing when they are present in 

 a liquid like brine that does not freeze. When the sub- 

 stance in which the bacteria are present becomes solid by 

 freezing, growth must cease but the bacteria are not 

 necessarily killed. For example water bacteria are not 

 all killed by freezing although it is popularly supposed 

 that water purifies itself in this manner. Even disease- 

 producing bacteria may resist this degree of cold for a 



