FACTORS OF PATHOGENICITY AND INFECTION 231 



rapid multiplication, progressively invading more and more exten- 

 sive areas of the animal tissues, while others may remain localized 

 at the point of invasion and exert their harmful action chiefly by 

 local growth and the elaboration of specific poisons. 



The inciting or inhibiting factors which permit or prohibit an 

 infection are dependent in part upon the nature of the invading 

 germ and in part upon the conditions of the defensive mechanism 

 of the subject attacked. 



Bacteria are roughly divided into two classes, saprophytes and 

 parasites. The saprophytes are those bacteria which thrive best on 

 dead organic matter and fulfill the enormously important function 

 in nature of reducing by their physiological activities the excreta 

 and dead bodies of more highly organized forms into those simple 

 chemical substances which may again be utilized by the plants in 

 their constructive processes. The saprophytes, thus, are of extreme 

 importance in maintaining the chemical balance between the animal 

 and plant kingdoms. Parasites, on the other hand, find the most 

 favorable conditions for their development upon the living bodies 

 of higher forms. 



While a strict separation of the two divisions cannot be made, 

 numerous species forming transitions between the two, it may be 

 said that the latter, class comprises most of the so-called pathogenic 

 or disease-producing bacteria. Strict saprophytes may cause dis- 

 ease, but only in cases where other factors have brought about the 

 death of some part of the tissues, and the bacteria invade the 

 necrotic areas and break down the proteins into poisonous chemical 

 substances such as ptomains, or through their own destruction give 

 rise to the liberation of toxic constituents of their bodies. It Is 

 necessary, therefore, that bacteria, in order to incite disease, should 

 belong strictly or facultatively to the class known as parasitic. It 

 must not be forgotten, however, that the terms are relative, and that 

 bacteria ordinarily saprophytic may develop parasitic and patho- 

 genic powers when the resisting forces of the invaded subject are 

 reduced to a minimum by chronic constitutional disease or other 

 causes. 



Organisms that are parasitic, however, are not necessarily patho- 

 genic, and there are certain more or loss fundamental requirements 

 which experience has taught us must be met by an organism in 

 order that it may be infectious (or pathogenic) for any given 

 animal; and by infectiousness is meant the ability of an organism 



