BACTERIAL RESPIRATION 211 



From a consideration of all these facts it can be 

 appreciated that it is not possible to define the classes 

 aerobic, facultative and strict anaerobic organisms at all 

 rigidly. For instance, it is not strictly accurate to define 

 aerobes as bacteria which need oxygen as hydrogen 

 acceptor, since we have seen that they will grow anaerobi- 

 cally if other suitable hydrogen acceptors are provided. 

 The converse also holds ; the normally strict anaerobe, 

 CI. sporogenes, can be made to grow in oxygen if a strongly 

 reducing substance like cystein is added to the medium. 

 The role played by the toxicity of oxygen must also be 

 borne in mind, particularly in considering micro -aerophilic 

 organisms which will only tolerate a lowered oxygen 

 tension in spite of the fact that they normally use oxygen 

 for their respiratory processes. It should be pointed 

 out that oxygen is not itself toxic, but gives rise to 

 hydrogen peroxide which is toxic. On the other hand, 

 the streptococci, which grow in the presence of oxygen 

 but do not utilise it for their respiration, are not on that 

 account called anaerobes, although their metabolism is 

 exactly that of the anaerobes. While oxygen may not be 

 required for the supply of energy, it is sometimes necessary 

 for the growth requirements of the organism. The urea 

 bacteria afford an illustration of this ; they derive their 

 energy needs by the breakdown of urea to ammonia 

 and carbon dioxide, but they cannot grow in the absence 

 of oxygen, because the synthesis of their cell constituents 

 involves dehydrogenation reactions, needing oxygen as 

 a hydrogen acceptor, urea itself being unable to serve 

 as such. 



Strictly speaking, then, aerobes and anaerobes are not 

 clear-cut groups, but may be more or less interchangeable 

 under appropriate conditions. The differentiation of 

 bacteria into aerobic, anaerobic and facultative organisms 

 is, however, a very useful working classification for 

 general purposes. 



A list of the cliief enzymes involved in the respiration 



