THE ENVIRONMENT OF BACTERIA. 69 



an organism is designated as a facultative anaerobe. On 

 the other hand, a germ which may grow in the presence of 

 air, but which grows best under anaerobic conditions, is 

 known as a facultative aerobe. The transition from one 

 extreme to the other can be seen from the following arrange- 

 ment of. the groups: 



Obligative aerobic bacteria, 

 Facultative anaerobic bacteria, 

 Facultative aerobic bacteria, 

 Obligative anaerobic bacteria. 



The ability of an organism to grow in the tissues and 

 fluids of the body indicates that it can thrive in the more 

 or less complete absence of free oxygen. Consequently, 

 most of the disease-producing bacteria are facultative ana- 

 erobes. When grown, however, under artificial conditions 

 they may show an almost obligative aerobic character. 

 This is especially true of the cholera vibrio. 



Although many bacteria can live in the absence of free 

 oxygen it must not be supposed that they are wholly inde- 

 pendent of that element. As stated heretofore, living pro- 

 toplasm always contains proteins and these compounds 

 contain oxygen as one essential constituent. Consequent- 

 ly, anaerobic bacteria require oxygen in some shape or 

 other. The fact that they obtain oxygen from some source 

 is seen not only in the chemical composition of the cell, 

 but also in the fact that anaerobes, like other bacteria, 

 Exhale carbonic acid. Apparently, anaerobic bacteria ob- 

 tain the necessary oxygen from various organic compounds. 

 The proteins can be utilized for this purpose. The carbo- 

 hydrates, especially glucose; are valuable in this respect. 

 It is customary, therefore, to grow anaerobic bacteria on 

 media to which glucose has been added. 



Obligative anaerobic bacteria are commonly met with 

 in the soil. They are present, to some extent, in the intes- 

 tinal contents. Only one organism of this group has been 



