22 GENERAL MORPHOLOGY AND BIOLOGY. 



that when it is transferred to fresh and good food supply 

 it does not multiply. It is during the process of degenera- 

 tion that the involution forms already mentioned occur. 

 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 vibrio, will not grow in the presence of the 

 smallest amount of free acid. 



Moisture. The presence of water is necessary for the 

 continued 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 to such hostile influences, even though they 

 survive, it by no means follows that they retain all their 

 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 

 ancerobes. 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 could be anaerobes, and those which are pre- 

 ferably anaerobes, but could be aerobes. As a matter of 

 fact such differences are manifested to a slight degree, 

 but all such organisms are usually grouped as facultative 

 anczrobes, i.e., preferably aerobic but capable of existing 

 without oxygen. Examples of obligatory aerobes are 



