28 BIOLOGY A 



Oxygen. Oxygen is obtained, by the large majority of bacteria, 

 directly from the atmosphere in the form of free O 2 . For many 

 microorganisms, moreover, the presence of free oxygen is a necessary 

 condition for growth. These are spoken of as the "obligatory 

 aerobes." Among the pathogenic bacteria proper, many, like the 

 gonococcus, bacillus influenza*, and bacillus pestis, show a marked 

 preference for a well-oxygenated environment. Probably there is 

 no pathogenic microorganism 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 infec- 

 tion 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 ferments 

 and proteolytic cleavage, from carbohydrates and proteins, or by 

 reduction from reducible bodies. Among the pathogenic microor- 

 ganisms 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 bac- 

 teria which thrive well, both under aerobic and anaerobic conditions. 

 Some of these, which have a preference for fre- _ D but never- 

 theless 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 vari- 

 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, 

 therefore, and not to be biologically dismissed by a rigid classifica- 

 tion into aerobes, facultative anaerobes, and obligatory anaerobes. 

 Both Engelmaiin, 4 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 macroscopic 



method of observing similar bacteria as to their motion away from 



* Er,,-jflmanfi r Botanisohe Zeitung, 1881. 

 nt. f. Bakt., I 



