36 BACTERIOLOGY 



of a proportion of common salt and peptone, the latter being 

 a soluble protein substance prepared by artificial digestion. 

 It does not coagulate on heating the fluid, and it is therefore 

 used to replace those protein substances which have been coagu- 

 lated and lost in the preparation of the meat-extract. For 

 full details of media-making reference must be made to larger 

 text-books. The nutrient broth so prepared may be used 

 alone ; or gelatin or agar 1 may be added to convert it into 

 a nutrient jelly. Glycerine, various sugars, and other sub- 

 stances are often added to facilitate the growth of the bacteria, 

 or to test the fermenting action of the bacteria upon such sub- 

 stances. Most organisms grow best in a neutral or faintly 

 alkaline medium, though a few prefer a faint acidity ; and 

 therefore the reaction of culture media is carefully adjusted, 

 litmus or some other indicator being used. 



Slices of potato and other vegetable tissues are often also 

 used, and bread-paste is a useful medium for growing moulds. 

 Numerous special media, often highly complex in their com- 

 position, are also used for special bacteria. Sometimes sub- 

 stances are added to prevent the growth of one type and help 

 the growth of another. 



Dead animal tissues, 2 partially or wholly immersed in suit- 

 able fluid, are also now used to grow special organisms upon, 

 e.g. the tubercle bacillus, and also certain of the disease-pro- 

 ducing Protozoa, and this method has recently led to some 

 important advances in our knowledge of such organisms, and 

 will probably lead to advances also in the immediate future in 

 our methods of diagnosis and treatment of many diseases. 

 Various observations of great importance can be made upon 

 the life-history of organisms under artificial cultivation. Some 

 grow easily, others only with difficulty or not at all. The 

 medium employed must be suitable, and for the more delicate 

 bacteria special substances or special media may be required. 

 Thus the influenza bacillus and some of its relatives require 

 the presence of haemoglobin or the red colouring matter of 

 the blood. The tubercle bacillus will not grow upon ordinary 

 media, but will grow upon egg or blood-serum, or upon other 

 media to which glycerine has been added. Some organisms 

 require the presence of oxygen, and are therefore known as 



1 Agar, or agar-agar to give it its full name, is a substance obtained 

 from certain seaweeds which grow in the Far East. It is used when a 

 jelly which does not melt too easily is required. 



2 It is interesting to notice in this connection that small pieces of 

 living animal tissues, if carefully removed and immersed in blood-serum, 

 may themselves survive in artificial ' ' culture " for several days or weeks, 

 and may even proliferate or grow in an irregular fashion. 



