EFFECT OF OXYGEN. 141 



which the cultures were grown and the use of silk 

 threads or cover-glasses. In all these experiments, of 

 course, it should be previously determined that in 

 spore-bearing species there are no spores present. 



Behavior Toward Oxygen and Other Gases. As already 

 noted under the nutritious substances required by bac- 

 teria, it is customary to divide bacteria into three classes, 

 according to their behavior toward oxygen. 



1 . Aerobic Bacteria. Growth only in the presence of 

 oxygen; the slightest restriction of air inhibits devel- 

 opment. Spore-formation especially requires the free 

 admission of air. 



2. Anaerobic Bacteria. Growth and spore-formation 

 only in the total exclusion of oxygen. Among this class 

 of bacteria are the bacillus of malignant oedema, the 

 tetanus bacillus, the bacillus of symptomatic anthrax, 

 and many soil bacteria. Exposed to the action of oxy- 

 gen, the vegetative forms of these bacteria are readily 

 destroyed; these spores, on the contrary, are very re- 

 sistant. Anaerobic bacteria being deprived of oxygen 

 the chief source of energy supplied to the aerobic spe- 

 cies, by which they oxidize the nutritive substances in 

 the culture media they are dependent for their nutri- 

 tion upon decomposable substances, such as grape- 

 sugar, which on separating into two smaller molecules 

 alcohol and carbonic acid give out energy or heat. 

 Anaerobic bacteria, therefore, require for their cultiva- 

 tion, as a rule, media containing 1 to 2 per cent, of 

 glucose or some equivalent. 



3. Facultative Aerobic and Facultative Anaerobic Bac- 

 teria. The greater number of aerobic bacteria, including 

 most of the pathogenic species, are capable of withstand- 

 ing, without being seriously affected, some restriction 



