34 BACTERIOLOGY. 



especially the motile forms, are also closely allied to 

 some of the micro-organisms which belong to the ani- 

 mal kingdom. If we exclude the micro-organisms 

 containing chlorophyll, bacteria may be defined as 

 extremely minute vegetable organisms; without chloro- 

 phyll, consisting of single spherical, rod-shaped, or 

 corkscrew-like cells or aggregates of such cells, between 

 whose protoplasm and nucleus it has been as yet im- 

 possible to differentiate with certainty. 



Bacteria occur as saprophytes or refuse-eaters and as 

 parasites. Saprophytic bacteria are such as commonly 

 exist independently of a living host, obtaining their 

 supply of nutriment from soluble food-stuffs in dead 

 organic matter. Parasitic bacteria, on the other hand, 

 live on or in some other organism, from which they 

 derive their nourishment for the whole or a part of 

 their existence. Those bacteria which depend entirely 

 upon a living host for their existence are known as 

 strict parasites ; those which can lead a saprophytic 

 existence, but which can also thrive within the body 

 of a living animal, are called facultative parasites. 

 The strict saprophytes, which represent the large 

 majority of all bacteria, while they destroy refuse, are 

 not only harmless to living organisms but perform 

 many important functions in nature without which 

 existence would be impossible, such as the destruction 

 of dead organic material through decomposition, putre- 

 faction, and fermentation. The parasites, on the con- 

 trary, though some of them may multiply in the secre- 

 tions or on the surface of the body without injury to 

 the animal upon which they depend for their exist- 

 ence, are usually harmful invaders, giving rise through 

 tHe lesions brought about in the body tissues by their 



