PHYSIOLOGY OF MICROORGANISMS 47 



fungi and the encysted cells of some protozoa will live for years, 

 while other forms are destroyed if allowed to become completely 



dried. 



RESPIRATION OF MICROORGANISMS 



Respiration is frequently defined as the taking up of oxygen 

 and the elimination of carbon dioxid. This definition is entirely 

 inadequate when we come to a discussion of microorganisms, 

 if, indeed, it can be applied in any case even to higher animals 

 and plants'. Respiration seems fundamentally to be the process 

 whereby energy is generated in the cell. Energy when evolved in 

 the cell always originates from chemical changes in the compounds 

 within the cell. Whether or not this energy may be gained by 

 the oxidation of food materials when taken into the cell, or whether 

 they must be first built up into protoplasm and this then broken 

 down, is a matter of dispute at present among scientists. In 

 any event the presence of free oxygen is certainly not neces- 

 sary to this release of energy, for many bacteria as well as other 

 plants and animals live in the absence of free oxygen. Organ- 

 isms that grow only in the presence of oxygen are called aerobic; 

 those which will grow only in the absence of free oxygen, anaerobic, 

 and those which will grow either with or without free oxygen, facul- 

 tative. It is probable that most of the so-called anaerobes grow 

 better in the presence of minute quantities of oxygen. The 

 end-products of respiration are found to differ with the type, 

 aerobic bacteria usually produce carbon dioxid and waterj anae- 

 robic forms, less highly oxidized substances, such as alcohol and 

 butyric acid. 



The oxygen requirements of anaerobic bacteria must be recog- 

 nized in the laboratory if they are to be successfully cultivated. 

 The air of the culture-tube or flask^jnay be removed by a stream 

 of hydrogen, nitrogen, or some other inert gas, the oxygen may be 

 absorbed by the use of alkaline sodium pyrogallate, the air may 

 be exhausted by an air-pump, the oxygen may be excluded by 

 covering the medium with oil or some similar material, or the 

 organism may be mixed with some aerobic form which will use 

 up the oxygen and allow growth of the anaerobe. Probably 

 the latter is the common method whereby anaerobes are able to 

 grow in nature. 



