CHAPTER II. 

 CULTURE MEDIA. 



Introduction. 



Section I. Liquid media, p. 30. 



1. Media made from animal tissues and fluids, p. 30. 2. Media made from vegetable 

 tissues, p. 37. 3. Synthetic media, p. 38. 

 Section II. Solid media, p. 39. 



1. Gelatin media, p. 39. 2. Agar media, p. 42. 3. Media made from albuminous 

 fluids and tissues, serum, egg, etc., p. 45. 4. Media made from vegetable tissue, 

 p. 55. 5. Coloured media, p. 56. 



THE substances requisite for the growth of micro-organisms may be obtained 

 by macerating, infusing or boiling tissues of animal or vegetable origin. 

 Saline solutions in which some carbo-hydrate is dissolved also supply all the 

 ingredients essential for a culture medium. 



Culture media are either solid or liquid. 



Chemically, like all other living cells, micro-organisms consist of organic and 

 inorganic nitrogen and mineral salts ; it is therefore necessary in order to grow a 

 micro-organism that these three classes of substances be made available, together 

 with oxygen which is an essential to the life of all living structures. [Finally, a 

 certain amount of moisture is absolutely necessary.] 



Micro-organisms are divided into two large groups, the members of one of which 

 derive their oxygen, like more highly organized structures, from the free oxygen 

 of the atmosphere, while the members of the other group cannot multiply in presence 

 of free oxygen, but obtain the oxygen they require by the decomposition of sub- 

 stances containing it (Pasteur). The former are known as the aerobic, the latter 

 as the anaerobic organisms. 



These two groups of micro-organisms call for very different methods of artificial 

 cultivation. Aerobic micro-organisms should be grown in vessels in which there 

 is an ample supply of air ; anaerobic micro-organisms on the other hand only 

 grow if air be excluded. The latter therefore are cultivated either in vacuo, or 

 in presence of some inert gas. 



The constituents of culture media are however the same for both aerobic and 

 anaerobic organisms and ought to include nitrogen compounds and salts of the 

 ternary bases. Many organisms can convert inorganic nitrogen (nitrates, etc.) 

 into organic nitrogen, while in some cases organisms will grow in purely inorganic 

 solutions provided these contain a small quantity of some carbohydrate such 

 as sugar. 



General Rules. Every culture medium therefore must 



(1) contain the substances necessary for the growth of the organism to be sown 

 [(2) be of suitable reaction] ; (3) have been previously sterilized ; (4) be con- 

 tained in vessels which afford protection from contamination from without. 



