18 GENERAL MORPHOLOGY AND BIOLOGY 



under which we grow cultures may be better than the natural 

 conditions. For while one of two species of bacteria growing 

 side by side may favour the growth of the other, it may also 

 in certain cases hinder it, and therefore, when the latter is 

 grown alone it may grow better. Most bacteria seem to 

 produce excretions which are unfavourable to their own 

 vitality, for, when a species is sown on a mass of artificial 

 food medium, it does not in the great majority of cases go on 

 growing till the 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 pro- 

 duces 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 surround- 

 ings, though it is found that there exists a considerable adapt- 

 ability 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 provided by the growth of other bacteria. 

 Amongst pathogenic bacteria the influenza bacillus must, outside 

 the animal body, almost necessarily be provided with haemo- 

 globin, and the growth of the gonococcus and the meningococcus 

 is much favoured if human serum be a constituent of a medium. 

 When the food supply of a bacterium fails, it degenerates and 

 dies. The proof of death lies in the fact that when it is trans- 

 ferred 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. 



