104 APPLIED BACTEEIOLOGY 



believed that when the real nature of y is discovered 

 it will be in our power to prevent to a great extent epi- 

 demics of all kinds. 



Methods of Bacterial Action. — When the pathogenic 

 bacteria invade the animal body, they may exert their 

 pernicious power in either or both of two ways. They may 

 by their excessive multiplication cause stopping up of the 

 minute capillaries, or they may produce substances called 

 toxins, which exert poisonous effects upon susceptible 

 animals. These toxins are the result of the metabolic 

 processes of the organisms ; they are in some cases secreted 

 in the intercellular tissue of the organism, and in others 

 excreted by them. 



The result of the infection of the body by the patho- 

 genic organisms varies very much in different cases, and 

 is manifested by the various symptoms which characterise 

 disease, such as disturbances of the nervous system, fever, 

 tissue changes, etc. With the lower animals various forms 

 of septicaemia may be produced, as in the case of rabbits 

 infected by the pneumococcus. In man septicaemia is not 

 usually produced, nor is there the same multiplication of 

 the organisms in the blood, although in some cases, notably 

 in that of the organism of relapsing fever, the multiplica- 

 tion of the organisms is liable to be exceedingly rapid. 

 Generally in the human subject the organisms remain 

 strictly local, as in the case of diphtheria and tetanus, or 

 proceed by the blood or lymphatics to various organs, 

 where they produce characteristic lesions, as in tubercu- 

 losis. 



The Antagonism of Micro-organisms (Symbiosis). — The 

 mutual antagonism or influence of growth of one species 

 upon the growth of another has been specially studied by 

 Freudenreich and Sirotinin. When several organisms are 

 associated in a liquid culture one species may take pre- 



