114 ESSAYS ON BACTEEIOLOGY. 



and tlie spirilla of cholera. The term bacterium, 

 though originally given to the rod or staff-like forms, 

 has now come to be applied to micro-organisms or 

 germs in general. 



Another important classification of bacteria is based 

 upon some marked biological property or activity. 

 Thus are described the color-producing or ,chromo- 

 genic organisms; the saprophytes, or those which live 

 upon dead organic material; the saprogenic or putre- 

 faction-producing bacteria; the zymogenic or ferment 

 producers, and hence the old term zymotic or ferment 

 diseases; and finally, to the physician the most im- 

 portant of all, the pathogenic or disease-producing 

 microbes. Flourescent and phosphorescent bacteria 

 are also described. These terms and groups are not 

 mutually exclusive, since, for example, bacteria may 

 be pathogenic and at the same time chromogenic or 

 color-producing. According to their ability to grow 

 with or without free oxygen, micro-organisms are also 

 said to be aerobic or anaerobic, a classification of great 

 practical as well as scientific importance. 



Many, perhaps most, bacteria are motionless, save 

 for a fine vibratory activity, the molecular or so-called 

 Brownian movement. Others have the power of true 

 locomotion, due to vigorous contractions and expan- 

 sions of their bodies, or to the possession of minute 

 appendages called flagella. At best, however, the 

 sphere of such locomotion is limited, and does not con- 

 fer the power of passing over great distances. Any 



