CHAPTER II. 





 ARTIFICIAL CULTIVATION OF BACTERIA. 



The bacteria are so small that many thousands of 

 them must 'be present in a mass before they can be seen 

 with the naked eye. The necessity of having masses of 

 bacteria of the same kind for study in the laboratory 

 thus becomes evident, and since such masses do not oc- 

 cur in nature it becomes necessary to grow them arti- 

 ficially. 



Food substances. The substances used in the labora- 

 tory upon which the bacteria can be grown are of ani- 

 mal or vegetable origin. These bacterial foods or * ' cul- 

 ture media, ' ' as they are technically called, may be pre- 

 pared from various vegetables, as potatoes or beets. 

 Broths made from meat or beef-extract are constantly 

 employed, as are milk, coagulated egg, and blood-serum. 

 The composition of these natural media are often modi- 

 fied through the addition of various qualifying sub- 

 stances, such as sugar, peptone, and glycerine, in order 

 to make them more suitable for the growth of certain 

 kinds of bacteria. 



All culture media when prepared from ordinary ma- 

 terials contain more or less bacterial life, because of the 

 presence of the organisms in the ingredients themselves, 

 and due to the inevitable contamination during the proc- 

 ess of manufacture. 



If the culture media are to be kept for any length of 

 time they must be freed from all living bacteria and 



