18 GENERAL MORPHOLOGY AND BIOLOGY 



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 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 

 three hours' drying, while the staphylococcus pyogenes aureus 

 will survive ten days' drying, and the bacillus diphtheriae 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 tosuch 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 

 called obligatory anaerobes. In still other bacteria the presence 

 or absence of oxygen is a matter of indifference. This group 

 might theoretically be divided into those which are preferably 

 aerobes, but can be anaerobes, and those which are preferably 

 anaerobes, but can be aerobes. As a matter of fact such 

 differences are manifested to a slight degree, but all such 

 organisms are usually grouped as facultative anaerobes, i.e. pre- 

 ferably aerobic but capable of existing without oxygen. Ex- 

 amples of obligatory aerobes are b. proteus vulgaris, b. subtilis ; of 

 obligatory anaerobes, b. tetani, b. oedematis maligni, w T hile the 

 great majority of pathogenic bacteria are facultative anaerobes. 

 With regard to anaerobes, hydrogen and nitrogen are indifferent 



