18 GENERAL MORPHOLOGY AND BIOLOGY 



growth. Such diffusion may be seen when the organism pro- 

 duces pigment, e.g., b. pyocyaneus. 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 

 frequently used. Some bacteria can exist on inorganic food, 

 but most require organic material to be supplied. Of the latter, 

 some require proteid to be present for their proper nourishment, 

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

 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. Occasionally 

 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 furnished by other bacteria. Again the 

 influenza bacillus must, outside the animal body, be provided 

 with fresh blood or serum, and the growth of the gonococcus, 

 and the meningococcus is much favoured if serum be a con- 

 stituent of a medium. The opinion has been expressed that 

 vitamines are provided by such media. 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 forms 

 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 

 thre'e hours' drying, while the staphylococcus pyogenes aureus 

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

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

 Anthrax spores will survive drying for several years, but here 

 again moisture enables them to resist longer than when they are 



