AEROBIOSIS AND ANAEROBIOSIS 61 



ceasa to grow long before a vacuum is reached (acetic bacteria, B. subtilis). 

 Contrasted with the obligatory aerobic bacteria, we have the obligatory- 

 anaerobic forms which thrive only in the absence of oxygen, small traces of 

 this gas being sufficient to inhibit growth (B. tetani, some butyric bacteria, 

 bacillus of malignant oedema). Between these extremes there is a great host 

 of bacteria representing every gradation between the two modes of life. 

 These are the facultative anaerobes, which, while growing best with a 

 plentiful supply of oxygen, are nevertheless able to exist with a very small 

 amount, and even with none at all, although in this case their vitality is often 

 much impaired. Anaerobic bacteria, both obligatory and facultative, are found 

 everywhere in nature where the air cannot penetrate, or where it is replaced 

 by other gases —in the deeper layers of the soil, for instance, in the mud of 

 rivers and standing waters, or the ooze of the sea bottom, and in manure. 

 In all such places anaerobic bacteria are the principal and often the only forms 

 of life, and by the fermentative and putrefactive processes they set up they 

 effect the disintegration and removal of dead animals and plants.* As be- 

 longing to the group of facultative anaerobic bacteria may be mentioned the 

 lactic acid ferment, the bacteria of typhoid and cholera, many staphylococci 

 and streptococci and most putrefactive bacteria. The power of anaerobic 

 growth varies, even in the same species, according to the source whence 

 the germs were derived and the mode of culture. Deprivation of oxygen 

 affects the bacteria in many ways. Some of the pigment bacteria (B. vio- 

 laceus, for instance) give rise to colourless cultures if oxygen be excluded ; 

 others, like Spirillum rubrum, form pigment most plentifully under these 

 conditions, though here again, as in so many other cases, exceptions occur. 

 Many obligatory anaerobic bacteria (e.g. some of the butyric ferments) 

 are motile, the energy necessary for movement being derived from the 

 partial breaking-down of the molecules of the fermentable substances on 

 which they grow. Access of oxygen stops the motion at once, just revers- 

 ing the conditions found among the aerobic forms, where the cilia work 

 the more vigorously the more oxygen is present, and become paralyzed at 

 once if it be cut off. These movements, minute as they are, represent an 

 expenditure of energy that is considerable when the size of the organisms 

 is taken into account. 



Those aerobic bacteria which possess the power of movement are 

 attracted by oxygen. If a drop of water containing them be placed upon 

 a slide and covered with a thin glass slip, they crowd to the edges of the 

 preparation and around air-bubbles in the water. This property of being 

 attracted by oxygen has been utilised by Engelmann (32) for the detec- 

 tion of minute traces of free oxygen. His ingenious method, which 



* The chemical aspects of the phenomena of putrefaction as well as the theoretical explanation 

 of anaerobiosis are discussed in Chap. XIV. 



