FOOD SUPPLY OF BACTERIA. 21 



grow cultures may be better than they naturally are. 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 ex- 

 cretions which are unfavourable to their own vitality, for 

 it is a frequent experience that, 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 

 probably diffuse out into the medium and prevent growth. 

 Evidence of such diffusion may be seen when the organism 

 produces pigment, which frequently can be observed in a 

 transparent medium far beyond the limit of the growth of 

 the organism, e.g., B. pyocyaneus growing on gelatine. 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. With some bacteria special 

 substances are necessary 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. While the result of the vital activity of bacteria 

 is in most cases to break up complex organic bodies into 

 simpler bodies, in certain cases a building-up process may 

 take place. This occurs in the sulphur bacteria just 

 mentioned, where the sulphur is oxidised into sulphates, 

 and also in the nitrifying bacteria of the soil, referred to 

 below (p. 29), which form nitrites and nitrates from 

 ammonia, and in some cases from the free nitrogen of the 

 air. When the food supply of a bacterium fails, it de- 

 generates and dies. The proof of death lies in the fact 



