146 MANUAL OF BACTERIOLOGY. 



CHAPTER V. 

 BACTERIA m DISEASE. 



To the physician and the student of medicine the study of 

 bacteriology is interesting chiefly on account of the great 

 importance attributed to bacteria in producing disease. The 

 presence in an organism of one or a number of organisms 

 of another species, which flourish as parasites upon the first, 

 is a phenomenon of very wide occurrence in nature. It is, in 

 fact, nearly universal. It may be observed among plants as 

 well as animals, for example in the familiar galls seen on some 

 of the higher plants, and mostly caused by the larvae of insects 

 harbored by the plant. We also find animals, such as tape- 

 worms and the trichina spiralis, living as parasites upon other 

 animals. The functions of the bacteria make them peculiarly 

 suited to leading a parasitic existence. The fact that they 

 possess no chlorophyll, and that they are therefore unable to 

 form carbon compounds from the carbon dioxide of the at- 

 mosphere, renders it necessary for them to secure such com- 

 pounds from preexisting organic matter. Most of them, 

 furthermore, flourish better when they are able to obtain nitrog- 

 enous food from organic matter rather than from inorganic 

 salts containing nitrogen. Most bacteria find the necessary 

 nutriment in the dead bodies of other animals and plants ; they 

 constitute what are known as saprophytes. But some of them 

 flourish upon the living bodies of other plants and animals in 

 whom they may produce disease. 



The phenomena of disease, we shall find, are very largely 

 due to the numerous waste products of the activities of bacteria, 

 which act as poisons to the host. 



