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 
