26 BIOLOGY AND TECHNIQUE 



organism which, under certain conditions of nutrition, is entirely 

 unable to exist and multiply in the complete absence of this gas. The 

 conditions existing within the infected animal organism cause it to 

 seem likely that all incitants of infection may, at times, thrive in the 

 complete absence of free oxygen. 



There is another class of organisms, on the other hand, for whose 

 development the presence of free oxygen is directly injurious. These 

 microorganisms, known as "obligatory anaerobes," obtain their supply 

 of oxygen indirectly, by enzymatic processes of fermentative and pro 

 teolytic cleavage, from carbohydrates and proteids, or by reduction 

 from reducible bodies. Among the pathogenic microorganisms the class 

 of " obligatory anaerobes " is represented chiefly by Bacillus tetani, the 

 bacillus of malignant edema, . the bacillus of symptomatic anthrax, 

 Bacillus aerogenes capsulatus, and Bacillus botulinus. 



Intermediate between these two classes is a large group of bacteria 

 which thrive well both under aerobic and anaerobic conditions. Some 

 of these, which have a preference for free oxygen but nevertheless 

 possess the power of thriving under anaerobic conditions, are spoken 

 of as "facultative anaerobes." In others the reverse of this is true; 

 these are spoken of as "facultative aerobes." These varieties of 

 bacteria are by far the most numerous and comprise most of our 

 parasitic and saprophytic bacteria. 



The relation of microorganisms to oxygen is extremely subtle, there- 

 fore, and not to be biologically dismissed by a rigid classification into 

 aerobes, facultative anaerobes, and obligatory anaerobes. Both Engel- 

 mann, 1 by a method of observing motile bacteria in the hanging drop 

 as to their behavior in relation to the oxygen given off by a chlorophyll- 

 bearing alga, and Beijerinck, 2 by a macroscopic method of observing 

 similar bacteria as to their motion away from or toward an oxygenated 

 area, were able to demonstrate delicately graded variations between 

 species, favoring various degrees of oxygen pressure. 



The discovery by Pasteur that certain bacteria develop only in the 

 absence of free oxygen, produced a revolution in our conceptions of 

 metabolic processes, since up to that time it was believed that life could 

 be supported only when a free supply of O 2 was obtainable. Pasteur's 

 original explanation for this phenomenon was that anaerobic conditions 

 of life were always associated with some form of carbohydrate fermenta- 



1 Engelmann, Botanische Zeitung, 1881. 

 Beijerinck, Centc f. Bakt., I, xiv, 1893. 



