RELATION TO GASEOUS ENVIRONMENT 17 



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

 bility 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 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. 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. 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 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 

 quite dry. When organisms have been subjected to such hostile 

 influences, even though they survive, it by no means follows that 

 they retain all their vital properties. 



Relation to Gaseous Environment. The relation of bacteria 

 to the oxygen of the air is such an important factor in the life 

 of bacteria that it enables a biological division to be made among 

 them. Some bacteria will only live and grow when oxygen is 

 present. To these the title of obligatory aerobes is given. Other 

 bacteria will only grow when no oxygen is present. These are 

 2 



