18 GENERAL MORPHOLOGY AND BIOLOGY. 



food supply is exhausted, but soon ceases to grow. Effete 

 products diffuse out into the medium and prevent growth. 

 Such diffusion may be seen when the organism produces 

 pigment, e.g. B. pyocyaneus growing on gelatin. In supplying 

 artificial food for bacterial growth, the general principle ought 

 to be to imitate as nearly as possible the natural surroundings, 

 though it is found that there exists a considerable adaptability 

 among organisms. With the pathogenic varieties it is usually 

 found expedient to use media derived from the fluids of the 

 animal body, and in cases where bacteria growing on plants are 

 being studied, infusions of the plants on which they grow are fre- 

 quently used. Some bacteria can exist on inorganic food, but 

 most require organic material to be supplied. Of the latter 

 some require for their proper nourishment proteid to be present, 

 while others can derive their nitrogen from such a non-proteid 

 as asparagin. All bacteria require nitrogen to be present in 

 some form, and many require to derive their carbon from 

 carbohydrates. Mineral salts, especially sulphates, chlorides, 

 and phosphates, and also salts of iron, are necessary. Occasion- 

 ally special substances are needed to support life. Thus some 

 species, in the protoplasm of which sulphur granules occur, 

 require sulphuretted hydrogen to be present. In nature the 

 latter is usually provided by the growth of other bacteria. When 

 the food supply of a bacterium fails, it degenerates and dies. 

 The proof of death lies in the fact that when it is transferred 

 to fresh and good food supply it does not multiply. If the 

 bacterium spores, it may then survive the want of food for a 

 very long time. It may here be stated that the reaction of the 

 food medium is a matter of great importance. Most bacteria 

 prefer a slightly alkaline medium, and some, e.g. the cholera 

 spirillum, will not grow in the presence of the smallest amount 

 of free acid. 



Moisture. The presence of water is necessary for the con- 

 tinued growth of all bacteria. The amount of drying which 

 bacteria in the vegetative stage will resist varies very much in 

 different species. Thus the cholera spirillum is killed by two or 

 three hours' drying, while the staphylococcus pyogenes aureus 

 will survive ten days' drying, and the bacillus diphtherias still 

 more. In the case of spores the periods are much longer. 

 Anthrax spores will survive drying for several years, but here 



